U.S. patent number 5,843,372 [Application Number 08/768,881] was granted by the patent office on 1998-12-01 for hydrogen-absorbing alloy for battery, method of manufacturing the same, and secondary nickel-metal hydride battery.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Hiroyuki Hasebe, Hiromichi Horie, Takamichi Inaba, Shusuke Inada, Yoshiyuki Isozaki, Yoshiko Kanazawa, Takao Sawa, Hiromi Shizu, Noriaki Yagi.
United States Patent |
5,843,372 |
Hasebe , et al. |
December 1, 1998 |
Hydrogen-absorbing alloy for battery, method of manufacturing the
same, and secondary nickel-metal hydride battery
Abstract
A hydrogen-absorbing alloy for battery according to the present
invention comprises an alloy having the composition represented by
a general formula A Ni.sub.a Mn.sub.b M.sub.c [where, A is at least
one kind of element selected from rare earth elements including Y
(yttrium), M is a metal mainly composed of at least one kind of
element selected from Co, Al, Fe, Si, Cr, Cu, Ti, Zr, Zn, Hf, V,
Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge and Sn, 3.5.ltoreq.a.ltoreq.5,
0.1.ltoreq.b.ltoreq.1, 0.ltoreq.c.ltoreq.1,
4.5.ltoreq.a+b+c.ltoreq.6], wherein the alloy has columnar
structures in which the area ratio of the columnar structures
having the ratio of a minor diameter to a major diameter (aspect
ratio) of 1:2 or higher is 50% or more. Further, an average minor
diameter of the columnar structures is set to 30 microns or less.
With this arrangement, there can be provided a nickel-metal hydride
battery capable of satisfying the three leading characteristics of
a high electrode capacity, long life and good rising-up all
together.
Inventors: |
Hasebe; Hiroyuki (Kawasaki,
JP), Inada; Shusuke (Yokohama, JP),
Isozaki; Yoshiyuki (Chigasaki, JP), Inaba;
Takamichi (Yokohama, JP), Sawa; Takao (Yokohama,
JP), Horie; Hiromichi (Yokosuka, JP), Yagi;
Noriaki (Yokohama, JP), Shizu; Hiromi (Fujisawa,
JP), Kanazawa; Yoshiko (Yokohama, JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Kawasaki, JP)
|
Family
ID: |
26390755 |
Appl.
No.: |
08/768,881 |
Filed: |
December 17, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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452544 |
May 30, 1995 |
5654115 |
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120412 |
Sep 14, 1993 |
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Foreign Application Priority Data
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Sep 14, 1992 [JP] |
|
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4-271121 |
Mar 11, 1993 [JP] |
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5-50295 |
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Current U.S.
Class: |
148/538; 429/59;
29/623.1; 429/101; 420/900 |
Current CPC
Class: |
H01M
10/345 (20130101); H01M 4/383 (20130101); C01B
3/0057 (20130101); B22F 9/10 (20130101); B22F
1/0055 (20130101); C22C 45/04 (20130101); B22F
2998/00 (20130101); Y10S 420/90 (20130101); Y02E
60/10 (20130101); Y02E 60/32 (20130101); H01M
2010/4292 (20130101); Y10T 29/49108 (20150115); B22F
2998/00 (20130101); B22F 1/0055 (20130101) |
Current International
Class: |
C01B
3/00 (20060101); B22F 9/08 (20060101); B22F
9/10 (20060101); H01M 10/34 (20060101); H01M
10/42 (20060101); C22C 014/03 () |
Field of
Search: |
;429/218,59,101,223,219,220,221,225 ;29/623.1,623.5 ;420/900 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2-220356 |
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Sep 1990 |
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JP |
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4-358008 |
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Dec 1992 |
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JP |
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5-156382 |
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Jun 1993 |
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JP |
|
Other References
Hudrogen storage alloys rapidly solidified by the melt-spinning
method and their characteristics as metal hydride electrodes,
Journal of Alloys and Compounds, Mishima et al., 192 (193) 176-178,
1993..
|
Primary Examiner: Nuzzolillo; M.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Parent Case Text
This is a Division of application Ser. No. 08/452,544 filed on May
30, 1995, now U.S. Pat. No. 5,654,115 which is a continuation of
application Ser. No. 08/120,412, filed on Sep. 14, 1993, abandoned.
Claims
What claimed is:
1. A method of manufacturing a hydrogen-absorbing alloy for a
battery, comprising the step of:
injecting a molten alloy having the composition represented by a
general formula A Ni.sub.a Mn.sub.b M.sub.c where A is at least one
element selected from the group consisting of rare earth elements
including Y (yttrium), M is a metal mainly composed of at least one
element selected from the group consisting of Co, Al, Fe, Si, Cr,
Cu, Ti, Zr, Zn, Hf, V, Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge and Sn,
3.5.ltoreq.a.ltoreq.5, 0.1.ltoreq.b.ltoreq.1, 0.ltoreq.c.ltoreq.1,
4.5.ltoreq.a+b+c.ltoreq.6 onto the traveling surface of a cooling
roll(s) rotating at a high speed; and
rapidly quenching and solidifying said molten alloy at a quenching
rate of 1800.degree. C./sec. or higher to prepare a
rapidly-quenched molten alloy to provide said hydrogen-absorbing
alloy for a battery, wherein said hydrogen-absorbing alloy has
columnar structures in which an area ratio of the columnar
structures having the ratio of a width to a length (aspect ratio)
of 1:2 or higher is 50% or more, and said columnar structures have
an average width of 30 microns or less.
2. A method of manufacturing a hydrogen-absorbing alloy for a
battery according to claim 1, wherein said molten alloy is rapidly
quenched in vacuum.
3. A method of manufacturing a hydrogen-absorbing alloy for a
battery, comprising the steps of:
rapidly quenching a molten alloy having the composition represented
by a general formula A Ni.sub.a Mn.sub.b M.sub.c where A is at
least one element selected from the group consisting of rare earth
elements including Y (yttrium), M is a metal mainly composed of at
least one element selected from the group consisting of Co, Al, Fe,
Si, Cr, Cu, Ti, Zr, Zn, Hf, V, Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge
and Sn, 3.5.ltoreq.a.ltoreq.5, 0.1.ltoreq.b.ltoreq.1,
0.ltoreq.c.ltoreq.1, 4.5.ltoreq.a+b+c.ltoreq.6 to prepare a
rapidly-quenched molten alloy; and
subjecting said obtained rapidly-quenched molten alloy to a heat
treatment at a temperature range of 200.degree.-500.degree. C. for
at least one hour for forming a hydrogen-absorbing alloy for a
battery, wherein said hydrogen-absorbing alloy has columnar
structures in which an area ratio of the columnar structures having
the ratio of a width to a length (aspect ratio) of 1:2 or higher is
50% or more, and said columnar structures have an average width of
30 microns or less.
4. A method of manufacturing a hydrogen-absorbing alloy for a
battery according to claim 3, wherein said rapidly quenched molten
alloy is formed by injecting a molten alloy onto the traveling
surface of a cooling roll(s) rotating at a high speed and rapidly
quenching the same and the peripheral speed of the traveling
surface of said cooling roll(s) is set to the range of 5-15
m/sec.
5. A method of manufacturing a hydrogen-absorbing alloy for a
battery according to claim 3, wherein said molten alloy is rapidly
quenched in vacuum.
6. A method of manufacturing a hydrogen-absorbing alloy for a
battery according to claim 3, wherein said heat treatment is
carried out in vacuum or an inert gas atmosphere.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydrogen-absorbing alloy for a
battery, a method of manufacturing the same and a nickel-metal
hydride battery using the alloy, and more specifically, to a
hydrogen-absorbing alloy for a battery capable of, when applied to
a negative electrode of the battery, satisfying all of the three
leading characteristics or a high electrode capacity (battery
capacity), long life (long cycle life) durable for repeated use and
excellent initial activity as well as a stabilized electric
potential (evenness of a voltage), a method of manufacturing the
same and a secondary nickel-metal hydride battery.
2. Description of the Related Art
Recently, the miniaturization and portability of electronic
appliances, which cannot be conventionally expected, has been
achieved by the progress of a power saving technology and mounting
technology realized by the progress of electronics. Under such a
circumstance, a secondary battery used as a power source of these
electronic appliances is required to have a large capacity and long
life. For example, in the field of office automation appliances,
telephone devices, audio/visual appliances having been developed
for personal use and portable use, the development of battery
having a high performance is desired to operate these appliance for
a longer time without using a power supply cable. Although a
non-sintering type nickel-cadmium battery having the electrode
substrate, which is composed of three-dimensional structure, of a
conventional sintering type nickel-cadmium battery has been
developed, the capacity of this battery has not been remarkably
increased.
Thus, there is recently proposed and highlighted a secondary
alkaline battery (secondary nickel-metal hydride battery) using the
hydrogen-absorbing alloy powder fixed to a collector as a negative
electrode. The electrode used to this nickel-metal hydride battery
is made by the following procedure. That is, hydrogen-absorbing
alloy is melted by a high frequency induction melting method, arc
melting method or the like and then cooled and pulverized and the
thus obtained pulverized powder is added with an electric
conductive agent and binder to form a kneaded material, and this
kneaded material is coated to or pressingly attached to a
collector. The negative electrode using the hydrogen-absorbing
alloy is characterized in that it can not only increase an
effective energy density per a unit weight or capacity but also has
a less amount of poisonous property and a less possibility of
environmental pollution as compared with cadmium used as a material
for the negative electrode of a conventional typical secondary
alkaline battery.
The negative electrode containing the hydrogen-absorbing alloy,
however, is immersed into a thick alkaline solution as a battery
electrolyte when it is assembled to a secondary battery as well as
exposed to oxygen evolved from a positive electrode when the
battery is excessively charged, and thus the hydrogen-absorbing
alloy is corroded and the electrode characteristics thereof are
liable to be deteriorated. Further, when the battery is charged,
hydrogen is absorbed into and released from the hydrogen-absorbing
alloy to cause the volume of the alloy to be expanded and shrank,
and thus cracks are produced to the hydrogen-absorbing alloy, by
which the pulverization of the hydrogen-absorbing alloy is
progressed. When the pulverization of the hydrogen-absorbing alloy
is progressed, the increase of the specific surface area of the
hydrogen-absorbing alloy is accelerated, and thus the ratio of the
surface area of the hydrogen-absorbing alloy deteriorated by the
alkaline battery electrolyte is increased.
Moreover, since the electric conductivity between the
hydrogen-absorbing alloy and the collector is also deteriorated, a
cycle life is shortened as well as the electrode characteristics
are also deteriorated.
To solve the above problems, although there are proposed such
methods as providing the hydrogen-absorbing alloy with a
multi-element structure, preventing the direct contact of the
hydrogen-absorbing alloy with the battery electrolyte by covering a
copper thin film or nickel thin film onto the surface of the
hydrogen-absorbing alloy powder or the surface of a negative
electrode containing the hydrogen-absorbing alloy by a plating
method, vapor deposition method or the like to improve the
corrosion resistance of the hydrogen-absorbing alloy, preventing
cracks by increasing the mechanical strength of the
hydrogen-absorbing alloy, or suppressing the deterioration of the
surface of the hydrogen-absorbing alloy by drying the same after it
has been immersed into an alkaline solution, these methods cannot
always achieve a sufficient improvement and sometimes lower an
electrode capacity on the contrary.
Further, as described above, although the electrode characteristics
of the conventional hydrogen-absorbing alloy are deteriorated by a
kind of corrosive reaction caused by the alkaline battery
electrolyte, the battery electrolyte is consumed by the reaction.
Therefore, the battery electrolyte in the conventional battery has
an amount larger than that necessary to smoothly cause a battery
reaction so that the battery reaction is not prevented even if the
amount of the battery electrolyte is reduced to some extent. When,
however, the amount of the electrolyte is increased and the surface
of the hydrogen-absorbing alloy electrode is covered with it, the
reaction speed for consuming an oxygen gas evolved in an
excessively charged region is lowered, and thus a problem arises in
that a battery internal pressure is increased.
Further, the aforesaid deterioration of the hydrogen-absorbing
alloy is also a problem when battery is designed.
That is, a secondary alkaline battery is designed to sealed in such
a manner that when the battery is discharged, a portion of the
hydrogen-absorbing alloy electrode usually remains in a charged
state and when the battery is charged, a portion of the
hydrogen-absorbing alloy electrode partially remains in an
uncharged state. Since, however, the hydrogen-absorbing alloy is
deteriorated with the progress of charge and discharge, a large
amount of the hydrogen-absorbing alloy must be contained so that
the above relationship can be maintained even if the alloy is
deteriorated in order to obtain a sufficient cycle life as a
battery. Consequently, since the volume of a nickel electrode as a
capacity-limiting electrode is reduced and the volume of the
hydrogen-absorbing alloy electrode is increased, the increase of
the battery capacity is prevented as well as a cost of the battery
is increased because the hydrogen-absorbing alloy is expensive.
Incidentally, the aforesaid hydrogen-absorbing alloy is composed
of, for example, AB.sub.2 type or A.sub.2 B type hydrogen-absorbing
alloy represented by Zr--Ti--Mn--Fe--Ag--V--Al--W, Ti.sub.15
Zr.sub.21 V.sub.15 Ni.sub.29 Cr.sub.5 Co.sub.5 Fe.sub.1 Mn.sub.8
and the like. These hydrogen-absorbing alloys are made by a usual
method of pulverizing an alloy made by being melted and cast. When
this series of alloys are used to a negative electrode, they
exhibit a high electrode capacity and provide a good capacity of
about 300 mAh/g, 400 mAh/g, respectively as well as almost all the
metal materials constituting the alloys are less expensive.
These alloys, however, have a drawback in that they are generally
difficult to be made to have a composition distributed uniformly.
Further, a battery using this series of alloys as an electrode
material has a drawback in that the battery has a delayed rising-up
of a capacity, and thus a high electrode capacity can be obtained
for the first time after an activating operation (charge/discharge
operation) of several tens of cycles is repeated after the battery
has been assembled. Moreover, this series of alloys also have a
drawback in that discharging characteristics are bad in a large
current and further a voltage greatly drops at a low temperature.
That is, this series of the alloys cannot achieve the high capacity
in the three leading characteristics or a high electrode capacity,
long life and excellent initial activity, whereas they cannot
satisfy the technical requirements in the aspect of the initial
activity (rising-up property).
On the other hand, there is an AB.sub.5 type alloy represented by
LaNi.sub.5 as another hydrogen-absorbing alloy used to secondary
alkaline battery. A negative electrode using this series of an
alloy having a hexagonal-crystal structure has the feature that it
can increase the effective energy density per a unit weight or unit
volume of a battery and a battery capacity as well as has a less
possibility to cause the environmental pollution due to cadmium and
the like and good battery characteristics when compared with the
case in which cadmium as a conventional typical negative electrode
material for secondary alkaline battery is used. Further, the
battery using the AB.sub.5 type alloy has an advantage that it can
discharge a large current. In this connection, an AB.sub.5 type
hydrogen-absorbing alloy composed of Mm--Ni--Co--Al alloy (Mm is
referred to as misch metal) has a low electrode capacity of less
than 200 mAh/g and a cycle life determined by charge/discharge is
about 400 cycles, which does not reach the level for satisfying the
electrode capacity and cycle life needed by the recent technical
requirements.
Thus, a technology of relatively increasing the content ratio of
the A site is also employed to increase the electrode capacity of
the battery using the AB.sub.5 type hydrogen-absorbing alloy.
According to this technology, although the electrode capacity can
be increased by about 30%, a drawback arises in that the
charge/discharge cycle life is shortened.
Further, there is also employed a technology for increasing the
amount of La content in misch metal (Mm: a mixture of rare earth
elements containing 10-50 wt % of La, 5-60 wt % of Ce, 2-10 wt % of
Pr, 10-45 wt % of Nd and the like) constituting the A component.
That is, it is possible to increase the electrode capacity by about
30% by using misch metal containing an reduced amount of a Ce
element and a relatively increased amount of La. In this case,
however, it is also difficult to increase the cycle life.
As described above, a hydrogen-absorbing alloy suitable for
secondary nickel-metal hydride battery for satisfying the electrode
capacity, cycle life, initial rising-up characteristics and
stability of electric potential required by the recent technical
level is not yet realized for practical use.
SUMMARY OF THE INVENTION
A first object of the present invention made to solve the above
problems is to provide a hydrogen-absorbing alloy for battery
capable of satisfying three leading characteristics of a high
electrode capacity, long life and good rising-up together, a method
of manufacturing the same and a secondary nickel-metal hydride
battery using the hydrogen-absorbing alloy.
A second object of the present invention is to provide a
hydrogen-absorbing alloy for battery particularly excellent in long
life characteristics when used as a negative electrode activating
material of a secondary nickel-metal hydride battery and a
secondary nickel-metal hydride battery using the hydrogen-absorbing
alloy.
A third object of the present invention is to provide a secondary
nickel-metal hydride battery having a high capacity and long life
and further made by low cost by using a hydrogen-absorbing alloy
having a less amount of deterioration and limiting the amount of a
battery electrolyte and an electrode capacity ratio.
To achieve the first object, the inventors selected an AB.sub.5
type hydrogen-absorbing alloy as an object to be studied, taking
into consideration the point that an electrode capacity can be
easily increased and the point that hydrogen can be
absorbed/released in the vicinity of an ordinary temperature and
ordinary pressure. Then, the inventors trially made
hydrogen-absorbing alloys having various compositions by
substituting the components of the AB.sub.5 type alloy for various
elements and changing manufacturing methods and studied the effect
of the compositions, manufacturing methods, heat treatment
conditions and the like of the hydrogen-absorbing alloys on battery
characteristics. As a result, a knowledge as described below was
obtained step by step.
First, an invention for achieving the first object will be
described. It is found that when a part of the AB.sub.5 type
hydrogen-absorbing alloy is substituted for Mn, an electrode
capacity can be greatly improved to about 280 mAh/g. The inventors
find, however, the fact that when a substituted amount of Mn
exceeds a predetermined amount, the corrosion resistance of the
hydrogen-absorbing alloy is lowered and the life characteristics of
a battery using the hydrogen-absorbing alloy are lowered on the
contrary.
More specifically, a problem is made clear that the corrosion
caused by a thick alkaline solution as the battery electrolyte of a
secondary alkaline battery is liable to progress particularly when
the solution coexists with oxygen produced when the battery is
excessively charged, and thus the battery characteristics are
deteriorated.
Then, the inventors studied the reason why the life characteristics
of the battery is lowered by the addition of Mn. When the elements
constituting various AB.sub.5 type alloy structures added with Mn
were analyzed by an X-ray microanalyzer, there was confirmed the
tendency that an amount of segregated Mn was increased in each
alloy structure with the increase in the substituted amount of Mn.
When assumed from this tendency, it was found that the reduced life
of battery progressed with the increase in the substituted amount
of Mn was mainly caused by the segregations of Mn.
That is, when a casting method having a low cooling capability as a
conventional method of manufacturing hydrogen-absorbing alloy is
used, since crystals are grown isotropic in a cooling process,
particle boundaries are liable to be made irregular in the
particles of a hydrogen-absorbing alloy as well as the alloy exists
in a liquid phase state for a long time, and thus segregations are
liable to be made to particle boundaries. Further, even if columnar
crystals are partially formed by using a casting method having a
high cooling capability, the crystal growth with columnar structure
in a minor diameter direction is progressed, segregation is
promoted and the corrosion resistance of an alloy is liable to be
lowered. Further, Mn has a feature that it is more embrittle than
other alloy-constituting elements. As a result, the segregations on
the particle boundaries act as a starting point of corrosion as
well as a mechanical strength is lowered by the segregations, and
thus the alloy is remarkably pulverized.
Further, it is supposed that the segregations in the
hydrogen-absorbing alloy are liable to form a local battery, Mn is
solved out into an alkaline battery electrolyte by the electric
erosive action thereof and Mn on the surface of the alloy is
changed to Mn(OH).sub.2 to thereby accelerate the corrosion of the
alloy, and thus the amount of hydrogen absorbed to the
hydrogen-absorbing alloy itself is reduced and a battery capacity
is lowered by electrode exfoliated from the hydrogen-absorbing
alloy by being corroded. Further, it is also supposed that the
pulverization of the alloy is accelerated by the reduced strength
of the particle boundaries caused by the segregation, and thus the
deterioration of battery characteristics is progressed with
age.
From the facts mentioned above, it can be expected to obtain a
hydrogen-absorbing alloy having a high capacity and long life by
reducing the segregations of Mn.
Thus, the following methods were executed by the inventors as a
trial to reduce the segregations of Mn:
(1) when an alloy material was melted, it was put into a crucible
after it had been pulverized as much as possible and sufficiently
mixed. The alloy material was relatively well mixed in a molten
state, however, segregations were formed in the size of from
several tens of microns to several hundreds of microns when
cooled;
(2) when the alloy material was melted, a resistance heating member
was not used but a high frequency induction heating apparatus was
used and the molten alloy was forcibly stirred, and in this case,
although the alloy was very uniformly stirred in a molten state,
segregations were formed in the size of from several tens of
microns to several hundreds of microns when cooled;
(3) when a molten alloy was cast, it was homogenized by lowering
the viscosity of the molten alloy by increasing the temperature
thereof as high as possible and in this case, however, segregations
were formed in the size of from several tens of microns to several
hundreds of microns when cooled; and
(4) after the molten alloy had been cast, it was subjected to a
heat treatment (e.g., at 1000.degree. C. for 8 hours) to reduce the
segregations, and in this case, although the effect of the heat
treatment was large, segregations of the size of several tens of
microns remained.
As described above, even if a processing was executed by any one of
the methods or by combining two or more of the methods, required
characteristics could not be sufficiently satisfied although
segregations were reduced.
As a result of the various studies of countermeasures for
preventing the segregations of Mn and the like effected by the
inventors, at first, the inventors employed a method of quenching
an molten alloy having a predetermined composition containing Mn at
a quenching rate of 1000.degree.-1200.degree. C./sec. which was
higher than that of a conventional casting method.
Although this method could reduce the segregations of Mn, it also
could not satisfy the required characteristics.
Thus, the experiment was further continued by employing a high
quenching rate of 1800.degree. C./sec. or more. As a result, it was
discovered that the result of the experiment was completely
different from that obtained from the low quenching rate in the
range of 1000.degree.-1200.degree. C./sec. More specifically, it
was discovered that when the quenching rate was 1800.degree.
C./sec. or higher, the maximum value of the Mn concentration
distributed in the alloy was 1.3 times or less the average value of
the Mn concentration in the entire alloy as well as the maximum
diameter of Mn segregated in the alloy was 0.5 micron or less.
Further, it was found that when the alloy having the above Mn
distribution was applied to secondary nickel-metal hydride battery,
the charge/discharge cycle life characteristics of the battery was
greatly improved without lowering capacity characteristics.
Here, first, the distribution of the Mn concentration will be
described. In the hydrogen-absorbing alloy in which the maximum
value of the Mn concentration distributed in the alloy exhibits a
value exceeding 1.3 times the average value of the Mn concentration
in the entire alloy, locations having a partially large Mn
concentration are scattered in the alloy.
When the alloy is used to the negative electrode of nickel-metal
hydride battery, a corrosive reaction is liable to be caused in
these locations and the deterioration of the hydrogen-absorbing
alloy itself is liable to be progressed. Further, when the
locations having the different Mn concentration exist as described
above, the degree of expansion/shrinkage of the volume of the
hydrogen-absorbing alloy caused by the hydrogen absorption/release
due to a battery reaction is partially different, and thus the
pulverization of the alloy is liable to be progressed by the stress
caused in the alloy. Therefore, the deterioration of the
hydrogen-absorbing alloy is further progressed by the increase in
the specific surface area resulting from the pulverization.
The above phenomenon is difficult to be caused in the
hydrogen-absorbing alloy in which the maximum value of the Mn
concentration distributed in the alloy exhibits a value 1.3 times
or less the average value of the Mn concentration in the entire
alloy. Consequently, it is supposed that when this alloy is applied
to a negative electrode, the progress of corrosion of the alloy can
be suppressed and the cycle life characteristics of the
nickel-metal hydride battery is improved.
FIG. 29 shows the relationship between the ratio of the maximum
value of the Mn concentration in the alloy to the average value of
the Mn concentration in the entire alloy and the cycle life of the
battery obtained from the experiment of the inventors.
The graph of FIG. 29 shows that a peculiar point exists in the
vicinity of the point where the maximum value of the Mn
concentration is 1.3 times the average value thereof. That is, the
inventors have discovered for the first time the fact that battery
life characteristics are greatly changed on the boundary of the
peculiar point where the maximum value of the Mn concentration
reaches 1.3 time the average value thereof.
Next, the maximum diameter of segregated Mn will be described.
Since the hydrogen-absorbing alloy in which the segregated Mn has a
maximum diameter of 0.5 micron has a large segregated portion, when
this alloy is used to a nickel-metal hydride battery, the
segregated point acts as the starting point of a corrosive reaction
and the deterioration of the alloy itself is liable to be
progressed. Further, when the alloy is expanded/shrank by the
hydrogen absorption/release caused by a battery reaction, since
stress is concentrated to the Mn segregated point, cracks are
liable to be caused from the segregated point and pulverization is
further progressed.
On the other hand, the above phenomenon is difficult to be caused
in the hydrogen-absorbing alloy in which the segregated Mn has a
maximum diameter 0.5 micron or less.
Therefore, it is supposed that when this alloy is applied to the
negative electrode of a nickel-metal hydride battery, the progress
of corrosion of the negative electrode is suppressed and thus the
cycle life characteristics of the nickel-metal hydride battery is
improved.
FIG. 30 shows the result of measurement of the maximum Mn diameter
segregated in the alloy and the cycle life of the electrode using
this alloy obtained by an experiment.
It is found from the graph shown in FIG. 30 that a peculiar point
exists in the vicinity of the location where the maximum value of
Mn segregated in the alloy is 0.5 micron. That is, the inventors
have discovered for the first time the fact that a battery life is
greatly changed on the boundary of the peculiar point where the
maximum diameter of segregated Mn is 0.5 micron.
As described above, the inventors have found for the first time
that the peculiar points exist between the distribution of the Mn
concentration and the maximum diameter of segregated Mn and the
required characteristics of battery and that hydrogen-absorbing
alloy must be prepared by quenching a molten alloy at a quenching
rate of 1800.degree. C./sec. or higher to achieve the uniformity of
the Mn concentration and the reduction of segregations, and thus
the above object cannot be achieved by the conventional low
quenching rate of about 1000.degree.-1200.degree. C./sec.
Further, a columnar crystal structure with a special or novel shape
could be made for the first time by the high quenching rate of
1800.degree. C./sec. or higher (refer to Table 11).
The columnar crystal structure with the novel shape is composed of
crystal particles in which the ratio of minor diameter to major
diameter (aspect ratio) of columnar crystal particles is 1:2 or
higher. Then, it has been discovered that when the ratio of fine
columnar crystals occupying in the cross section of the pulverized
particles of the alloy is 50% or higher, preferably 70% or higher
and more preferably 80% or higher, a higher electrode capacity and
longer life cycle can be achieved at the same time. That is, it has
been discovered for the first time that when the above fine
columnar crystal structure is formed, the high electrode capacity
of 240 mAh/g or more and long cycle life of 500 times or more are
achieved at the same time.
That is, as a result of the various studies effected by the
inventors, it has been discovered that the characteristics of the
hydrogen-absorbing alloy as a negative electrode material can be
greatly improved by forming fine crystal particles having a
predetermined feature in the alloy by a rapidly-quenched method
from the melt at a quenching rate of 1800.degree. C./sec. or
higher.
As a result of the examination effected by the inventors, the
hydrogen-absorbing alloy made by the rapidly-quenched method from
the melt can effectively prevent the segregations of Mn and the
like.
Further, the inventors have obtained the knowledge that a
hydrogen-absorbing alloy for battery more excellent in
characteristics can be obtained by removing the fine internal
distortion produced by a rapid quenching process, by taking
attention to the internal distortion. More specifically, when an
alloy material in a molten state is rapidly quenched by the above
method, the quenching is rapidly executed by forming many crystal
nuclei, different from a casting method, and thus distortion is
liable to be made in the interior of the alloy by the quenching. It
is found that hydrogen is difficult to enter into the interior of
such alloy and to exit therefrom due to the formation of the
internal distortion, by which battery characteristics are
degraded.
Thus, as a result of various studies, the inventors have obtained
the knowledge that in the case of the hydrogen-absorbing alloy
according to the present invention having formed once the fine
crystal structure by the rapidly-quenched method from the melt, the
internal distortion can be removed while keeping the uniformity of
the alloy by only subjecting the alloy to a heat treatment within
the temperature range of 200.degree.-500.degree. C. which is far
lower than a recrystallizing temperature for a short time, thus a
hydrogen-absorbing alloy for battery more excellent in the
characteristics can be obtained.
More specifically, it has been found that the mitigation of the
internal distortion by the heat treatment enables hydrogen to more
easily enter into and exit from the interior of the alloy so that
the characteristics of a negative electrode material can be further
improved. At least one hour is necessary for the heat
treatment.
The present invention has been completed based on the above various
knowledges and findings. That is, a first hydrogen-absorbing alloy
according to the present invention comprises an alloy having the
composition represented by a general formula A Ni.sub.a Mn.sub.b
M.sub.c [where, A is at least one kind of element selected from
rare earth elements including Y (yttrium), M is a metal mainly
composed of at least one kind of element selected from Co, Al, Fe,
Si, Cr, Cu, Ti, Zr, Zn, Hf, V, Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge
and Sn, 3.5.ltoreq.a.ltoreq.5, 0.1.ltoreq.b.ltoreq.1,
0.ltoreq.c.ltoreq.1, 4.5.ltoreq.a+b+c.ltoreq.6], wherein the alloy
has a columnar structure in which the area ratio of columnar
structure having the ratio of a minor diameter to a major diameter
(aspect ratio) of 1:2 or higher is 50% or more. More specifically,
an electrode capacity is increased by substituting a portion of the
B component of an AB.sub.5 type hydrogen-absorbing alloy for Mn as
well as a hydrogen-absorbing alloy with a novel shape capable of
forming an electrode having a long cycle life is formed.
Further, when the above alloy is made by the rapidly quenching
process, the rapidly-quenched molten alloy preferably has a
thickness set to 10-150 microns. In addition, the columnar crystals
preferably have an average minor diameter of 30 microns or
less.
A second hydrogen-absorbing alloy according to the present
invention comprises an alloy having the composition represented by
a general formula A Ni.sub.a Mn.sub.b M.sub.c [where, A is at least
one kind of element selected from rare earth elements including Y
(yttrium), M is a metal mainly composed of at least one kind of
element selected from Co, Al, Fe, Si, Cr, Cu, Ti, Zr, Zn, Hf, V,
Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge and Sn, 3.5.ltoreq.a.ltoreq.5,
0.1.ltoreq.b.ltoreq.1, 0.ltoreq.c.ltoreq.1,
4.5.ltoreq.a+b+c.ltoreq.6], wherein when the characteristic X-ray
intensity of Mn contained in the alloy is observed by an X-ray
microanalyzer in the respective unit regions of the alloy obtained
by vertically and horizontally dividing into 100 portions the
observation regions of the alloy each composed of a cross sectional
area of 20 microns.times.20 microns, the maximum value among the
characteristic X-ray intensities of Mn in the respective
observation regions is 1.3 times or less the average value of the
characteristic X-ray intensities of Mn in the respective
observation regions. The aforesaid value of (the maximum value of
the Mn concentration in the alloy)/(the average value of the Mn
concentration in the alloy) can approximate (the maximum value
among the characteristic X-ray intensities of Mn in the respective
unit regions)/(the average value of the characteristic X-ray
intensities of Mn in the respective unit regions) when observed by
the X-ray microanalyzer in the respective unit regions of the alloy
obtained by vertically and horizontally dividing into 100 portions
the observation regions of the alloy each composed of a cross
sectional area of 20 microns.times.20 microns. In the present
invention, this value is set to 1.3 or less, and more preferably
set to 1.2 or less.
Further, a third hydrogen-absorbing alloy according to the present
invention comprises an alloy having the composition represented by
a general formula A Ni.sub.a Mn.sub.b M.sub.c [where, A is at least
one kind of element selected from rare earth elements including Y
(yttrium), M is a metal mainly composed of at least one kind of
element selected from Co, Al, Fe, Si, Cr, Cu, Ti, Zr, Zn, Hf, V,
Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge and Sn, 3.5.ltoreq.a.ltoreq.5,
0.1.ltoreq.b.ltoreq.1, 0.ltoreq.c.ltoreq.1,
4.5.ltoreq.a+b+c.ltoreq.6], wherein the maximum diameter of the Mn
particles segregated in the alloy is 0.5 micron or less.
A first manufacturing method of a hydrogen-absorbing alloy for
battery according to the present invention comprises the step of
injecting a molten alloy having the composition represented by a
general formula A Ni.sub.a Mn.sub.b M.sub.c [where, A is at least
one kind of element selected from rare earth elements including Y
(yttrium), M is a metal mainly composed of at least one kind of
element selected from Co, Al, Fe, Si, Cr, Cu, Ti, Zr, Zn, Hf, V,
Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge and Sn, 3.5.ltoreq.a.ltoreq.5,
0.1.ltoreq.b.ltoreq.1, 0.ltoreq.c.ltoreq.1,
4.5.ltoreq.a+b+c.ltoreq.6] onto the traveling surface of a cooling
roll(s) rotating at a high speed and rapidly quenching and
solidifying the molten alloy at a quenching rate of 1800.degree.
C./sec. or higher to form the hydrogen-absorbing alloy.
A second manufacturing method of a hydrogen-absorbing alloy for
battery according to the present invention comprises the steps of
rapidly quenching a molten metal having the aforesaid composition
at a quenching rate of 1800.degree. C./sec. or higher and
subjecting the thus obtained rapidly-quenched molten alloy to a
heat treatment at the temperature range of from
200.degree.-500.degree. C. for at least one hour to form a
hydrogen-absorbing alloy for battery.
Further, the rapid quenching process of the molten alloy is
preferably executed in vacuum or an atmosphere of an inert gas such
as Ar. In addition, the heat treatment is preferably executed in
vacuum or an inert gas atmosphere.
In the first to third hydrogen-absorbing alloys for battery
according to the present invention, when Ni.sub.a Mn.sub.b M.sub.c
are represented by B, the alloy composition according to the
present invention is AB.sub.4.6 -AB.sub.6 from
4.5.ltoreq.a+b+c.ltoreq.6.
When the composition ratio x of B (i.e., the value of a+b+c) is
other than the above range, the amount of phases (for example,
phases composed of AB, AB.sub.2, AB.sub.3, A.sub.2 B.sub.7 and the
like and a phase composed of a single element constituting a B
site, [hereinafter, referred to as a second phase]) other than
AB.sub.4.5 -AB.sub.6 created in the alloy is increased.
When the amount of the second phase other than AB.sub.x is
increased in the alloy, the ratio of the alloy phases which have
two or more different compositions including the second phase and
come into contact with one another is increased in the
hydrogen-absorbing alloy. The boundaries of the alloy phases having
the different compositions are weak in a mechanical strength and
cracks are liable to be made from these boundaries by the
absorption/release of hydrogen.
Further, segregations are liable to be made to the boundaries and
the hydrogen-absorbing alloy is liable to be corroded from the
segregations. Further, the second phase absorbs a less amount of
hydrogen as compared with AB.sub.x when used as an electrode, and
thus when an alloy including a large amount of the second phase as
the electrode, an electrode capacity per a unit volume is reduced.
In any way, when the hydrogen-absorbing alloy is used as an
electrode material, it reduces the electrode capacity and life.
After all, the reason why the value of x is limited is as follows.
When x is less than 4.5, a hydrogen-absorbing alloy which is less
corroded when a battery is charged/discharged and difficult to be
cracked and pulverized cannot be obtained. On the other hand, when
x exceeds 6, the creation of the second phase is admitted in an
alloy making method which can be usually employed in the industry
and thus the characteristics of the hydrogen-absorbing alloy cannot
be improved.
Therefore, although the value of x or (a+b+c) is set within the
range of 4.5-6, it is preferably 4.6-5.6 and more preferably within
the range of 5.05-5.5.
The component A constituting A Ni.sub.a Mn.sub.b M.sub.c according
to the present invention shows at least one kind selected from rare
earth elements including Y (specifically, Y, La, Ce, Pr, Nd, Pm,
Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). Note, since rare earth
elements of high purity or a rare earth element as a single element
are very expensive. Thus, the material cost of the
hydrogen-absorbing alloy can be greatly reduced by using a misch
metal (hereinafter, abbreviated as Mm or Lm) as a mixture of a
plurality of rare earth elements. A composition containing La of
10-50 wt %, Ce of 5-60 wt %, Pr of 2-10 wt %, Nd of 10-45 wt % is
usually used as the Mm.
Further, the reason why the value of the composition ratio a of Ni
is limited within the range of 3.5-5 is as follows. When the value
a is set to less than 3.5, the electrode capacity is lowered,
whereas when the value a exceeds 5, the mixing ratio of other alloy
components is relatively lowered and thus the capacity is difficult
to be increased.
Further, since Mn is effective to increase the capacity of the
negative electrode containing the hydrogen-absorbing alloy and
reduce a hydrogen absorption/release pressure (equilibrium
pressure), it is used as an essential element constituting the
alloy of the present invention. Mn is added within such a range
that the constitution ratio b thereof is 0.1-1.0. When the
constitution ratio b exceeds 1, the alloy electrode is liable to be
pulverized and its cycle life is shortened, and thus the upper
limit of the constitution ratio b is set to 1. On the other hand,
when the constitution ratio b of Mn is less than 0.1, the
improvement of the electrode capacity as one of the objects of the
present invention cannot be achieved.
Further, the component M in a general formula shows a metal mainly
composed of at least one kind of element selected from Co, Al, Fe,
Si, Cr, Cu, Ti, Zr, Zn, Hf, V, Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge
and Sn. The elements Co, Al, Fe, Si, Cr, Cu of the component M are
particularly effective to extend the life of the hydrogen-absorbing
alloy. The M component is added within such a range that the
constitution ratio c thereof is 1 or less. When the constitution
ratio c exceeds 1, the capacity of the electrode formed of the
alloy lowered, and thus the upper limit of the constitution ratio c
is set to 1.
Further, when the alloy is made by using a rapidly-quenched method
from the melt, a life can be extended to some extent because
segregations are prevented. Therefore, the lower limit of the
composition ratio c of the component M is set to 0.
Al of the component M has a function for lowering the hydrogen
absorbing/releasing pressure (dissociation) in the same way as Mn
as well as can increase durability.
Further, Co of the component M is effective to improve the
corrosion resistance of the alloy against a battery electrolyte and
the like and the pulverization of the alloy is greatly suppressed
by it. More specifically, when the substituted amount of Co is
increased, the cycle life is increased but there is the tendency
that the electrode capacity and the high rate discharge ability are
lowered, and thus the substituted amount of Co must be optimized in
accordance with the application of battery.
In addition to the above, the hydrogen-absorbing alloy according to
the present invention may contain at least one kind of element
selected from Pb, C, N, O, F Cl, S, P and the like as impurities in
the range by which the characteristics of the alloy of the present
invention is not obstructed.
The content of the impurities is preferably in the range of 6000
ppm or less, more preferably 5000 ppm or less and still more
preferably 4000 ppm.
Although a method of manufacturing the hydrogen-absorbing alloy for
battery according to the present invention is not particularly
limited so long as it can make a uniform alloy composition, prevent
segregations and obtain the crystal structure according to the
present invention, the hydrogen-absorbing alloy can be stably
manufactured in a large amount by using a
molten-metal-rapidly-quenching method such as a single roll method,
double roll method and the like to be describe below in detail with
reference to drawings and optimizing the material of a cooling
roll(s), rotating speed of the cooling roll(s) (peripheral speed of
the traveling surface thereof), a molten alloy temperature, kind of
gas in a cooling chamber, pressure, amount of the molten alloy to
be injected.
Single Roll Method
FIG. 1 shows a hydrogen-absorbing alloy manufacturing apparatus
using the single roll method. This manufacturing apparatus
comprises a cooling roll 5 composed of copper, nickel or the like
excellent in thermal conductivity and having a diameter of about
300 mm and a molten metal injection nozzle 4 for injecting a molten
hydrogen-absorbing alloy 3 supplied from a ladle 2 to the traveling
surface of the cooling roll 5 after storing the same. The cooling
roll 5 and the like are accommodated in a cooling chamber 1
adjusted to vacuum or an inert gas atmosphere. Further, although
the rotating speed of the cooling roll 5 depends upon the wetting
property thereof, cooling speed and an injection amount of the
molten hydrogen-absorbing alloy, it is generally set to 300-5000
rpm.
In the aforesaid manufacturing apparatus shown in FIG. 1, when the
molten hydrogen-absorbing alloy 3 supplied from the ladle 2 is
injected onto the traveling surface of the cooling roll 5 through
the molten metal injection nozzle 4, the molten alloy is solidified
from the surface thereof in contact with the cooling roll 5,
crystals begin to be grown and the solidification of the molten
metal is perfectly completed before it leaves from the cooling roll
5. Thereafter, the molten metal is further cooled while it flies in
the cooling chamber 1 so that a hydrogen-absorbing alloy 6 is made
which has the uniform concentration of constituting elements, a
less amount of segregations and crystals grown in the same
direction.
Double Roll Method
FIG. 2 shows a hydrogen-absorbing alloy manufacturing apparatus
using the double roll method. This manufacturing apparatus
comprises a pair or more of cooling rolls 5a, 5b disposed in a
cooling chamber 1 so that the respective traveling surfaces thereof
are confronted to each other, a melting furnace 7 for preparing a
molten hydrogen-absorbing alloy 3 by melting material metals, and a
molten metal injection nozzle 4 for injecting the molten
hydrogen-absorbing alloy 3 supplied from the melting furnace 7
between the cooling rolls 5a, 5b through a tundish 8.
The cooling rolls 5a, 5b are composed of a material excellent in
thermal conductivity such as copper, nickel or the like and having
a diameter of about 50 mm. These cooling rolls 5a, 5b are rotated
at a high speed of about 300-2000 rpm while keeping a fine gap d of
about 0-0.5 mm therebetween.
Note, although traveling surfaces of the cooling rolls are parallel
to each other as shown in FIG. 2, a so-called shape roll in which
the cross section of the traveling surface thereof is formed to a
U-shape or V-shape may be employed. Further, when the gap d between
the cooling rolls 5a, 5b is excessively large, since the molten
alloy is not quenched in the same direction and as a result a
hydrogen-absorbing alloy having a columnar structure whose growing
direction is disturbed is made, the gap d is preferably set to 0.2
mm or less. Further, when the gap d is excessively large, since a
quenching rate is lowered and the segregations of Mn are
accelerated, the uniformity of the Mn concentration is lowered and
a hydrogen-absorbing alloy having segregated particles of Mn grown
to a large size on particle boundaries is made, and thus the gap d
is set to 0.2 mm or less.
In the aforesaid manufacturing apparatus shown in FIG. 2, when the
molten hydrogen-absorbing alloy 3 is injected in the direction
between the cooling rolls 5a, 5b from the injection nozzle 4, it is
solidified from the sides thereof in contact with the cooling roll
5a, 5b on the both sides, crystals begin to be grown and the
solidification of the molten metal is perfectly completed before it
leaves from the cooling rolls 5a, 5b. Thereafter, the molten metal
is further quenched while it flies in the cooling chamber 1 so that
a hydrogen-absorbing alloy 6 is made which has a less amount of
segregations and a columnar structure according to the present
invention.
Rotating Disc Method
FIG. 32 shows a hydrogen-absorbing alloy particles manufacturing
apparatus using a rotating disc method. This manufacturing
apparatus comprises a rotary disc member 9 as a high speed rotary
member disposed in a cooling chamber 1 in an argon gas atmosphere
and a molten metal injection nozzle 4 for temporarily storing a
molten hydrogen-absorbing alloy supplied from a ladle 2 and further
injecting the same onto the traveling surface of the rotary disc
member 9. The rotary member 9 is composed of a ceramics or metal
material having a relatively low wetting property to a molten metal
to prevent the molten hydrogen-absorbing alloy 3 from adhering to
and solidifying on the rotary member 9.
In the manufacturing apparatus shown in FIG. 32, when the molten
hydrogen-absorbing alloy 3 supplied from the ladle 2 is injected
onto the traveling surface of the rotary disc member 9 from the
molten metal injection nozzle 4, it is finely dispersed by the
moving force of the rotary member 9 and spheroidized by the surface
tension of itself while frying in the cooling chamber 1 without
coming into contact with the inner surface of the cooling chamber 1
and further solidified by being quenched by the atmosphere gas such
as the argon gas or the like. As a result, hydrogen-absorbing alloy
particles 6 each having a spherical shape covered with a free
cooling surface are made. The hydrogen-absorbing alloy particles 6
are collected into a particle collection vessel 10 disposed on the
bottom of the cooling chamber 1.
Gas Atomizing Method
FIG. 33 shows a hydrogen-absorbing alloy particle manufacturing
apparatus using a gas atomizing method. This manufacturing
apparatus comprises a melting furnace 24 for heating and melting a
metal material disposed in a cooling chamber 1 in an argon gas
atmosphere by a heater 23 and preparing a molten hydrogen-absorbing
alloy 3, a molten metal injection nozzle 4 formed on the bottom of
the melting furnace 24 and having an inner diameter of about 2 mm,
a plurality of inert gas nozzles 25 disposed in the vicinity of the
lower end opening of the molten metal injection nozzle 4 in
confrontation therewith to inject an cooling inert gas such as an
argon gas or the like, and a shut-off valve 26 for opening/closing
the molten metal injection nozzle 4.
In the manufacturing apparatus shown in FIG. 33, when the argon gas
is supplied to the melting furnace 24 in which the molten
hydrogen-absorbing alloy 3 is accommodated, the liquid surface of
the molten alloy 3 in the melting furnace 24 is pressurized and the
molten alloy 3 is injected from the front end opening of the molten
metal nozzle 4 on the bottom of the melting furnace 24. At this
time, the inert gas nozzles 25, which are disposed substantially
perpendicularly to the direction in which the molten alloy 3 is
injected, inject the inert gas such as the argon gas or the like
toward the injected molten alloy at a high speed. With this
operation, the molten hydrogen-absorbing alloy 3 is atomized and
dispersed by the inert gas in the cooling chamber 1 without coming
into contact with the inner wall thereof and quenched and
solidified while being flown downwardly along the revolution flow
of the inert gas. As a result, hydrogen-absorbing alloy particles 6
each having a spherical shape covered with a free cooling surface
are made.
When a ribbon-shaped, flake-shaped or particle-shaped
hydrogen-absorbing alloy is made by using the aforesaid
molten-metal-rapidly-quenching method, equi-axed crystals and
columnar structure are made in an alloy structure depending upon
the conditions of the material of the cooling roll and rotary disk,
quenching rate of the molten alloy and the like.
The first to third hydrogen-absorbing alloys of the present
invention are suitably obtained by rapidly quenching molten alloy
particularly at a quenching rate of 1800.degree. C./sec. or higher.
Further, when the hydrogen-absorbing alloy is made by rapidly
quenching the molten alloy at the quenching rate of 180.degree.
C./sec. or higher, the respective crystal particles constituting
the alloy are finely crystallized to about 1-100 microns so that
the strength of the alloy is increased and the disturbance of
particle boundaries is reduced, and thus an amount of hydrogen to
be absorbed is increased and the electrode capacity can be
increased.
The above columnar structure is particularly developed in the first
hydrogen-absorbing alloy for battery according to the present
invention.
It has been confirmed by the experiment effected by the inventors
that since the columnar structure has crystals grown in the same
direction different from those in the equi-axed crystal structure,
grain boundaries are less disturbed, an amount of hydrogen to be
absorbed is increased and the electrode capacity can be increased.
More specifically, in the columnar structure, since the paths of
hydrogen molecules or hydrogen atoms are formed along the
boundaries of the columnar structure, hydrogen can be easily
absorbed to and released from the alloy to thereby increase the
electrode capacity. Further, segregations are greatly reduced in
the columnar structure. Therefore, a local battery is not formed by
the segregations and the reduction of life due to the pulverization
of the alloy can be effectively prevented.
In the crystal structure of the hydrogen-absorbing alloy made by
the rapidly-quenched method from the melt, the area ratio of the
columnar structure must be 50% or more, preferably 70% or more and
more preferably 80% or more in the cross section in the thickness
direction of the hydrogen-absorbing alloy, from the view point of
increasing battery characteristics when the alloy is assembled to a
battery as a hydrogen-absorbing alloy electrode. When the area
ratio of columnar structure is 50% or more, the cycle life of a
negative electrode using the alloy is extended than that of a
negative electrode using a hydrogen-absorbing alloy made by a
casting method. When the rapidly-quenched molten alloy is entirely
formed of columnar structure, segregations are particularly reduced
so that the capacity and life of an alloy electrode can be further
improved. On the other hand, when the area ratio is less than 50%,
there is no remarkable difference between the cycle life of the
negative electrode using the above alloy and that of the negative
electrode using the alloy made by the casting method. That is, the
excellent battery characteristics that the electrode capacity is
240 mAh/g or more and the cycle life is 500 times or more can be
simultaneously obtained by using the hydrogen-absorbing alloy of
the present invention made by the above
molten-metal-rapidly-quenching method and having the area ratio of
the columnar structure of 50% or more in the cross section in the
thickness direction of the alloy. The more preferable value of the
electrode capacity is 250 mAh/g or more and the still more
preferable value thereof is 255 mAh/g. Further, the more preferable
value of the cycle life is 550 times or more and the still more
preferable value thereof is 600 times or more.
Here, the columnar structure is defined as columnar crystal
particles having a ratio of minor diameter to major diameter
(aspect ratio) of 1:2 or higher.
The method of manufacturing the hydrogen-absorbing alloy according
to the present invention will be described in more detail.
The hydrogen-absorbing alloy for battery having the developed
columnar structure and a reduced amount of the segregations of Mn
and the like as described above is made by strictly controlling the
conditions for preparing and quenching a molten alloy in the single
roll method or double roll method or the like.
More specifically, although the molten alloy may be prepared by
accommodating a hydrogen-absorbing alloy (mother alloy) having the
above composition previously made by the casting method in a
melting crucible and melting the same by high frequency induction
heating, it is also possible that respective constituting elements
are directly put into a crucible after the substituted amounts
thereof have been adjusted to prepare the molten alloy without
using the mother alloy. At this time, it is preferable to add some
elements such as, for example, rare earth elements and Mn having a
high vapor pressure in a slightly greater amount in the elements
constituting the hydrogen-absorbing alloy. That is, an adjustment
is preferably made so that the change of the alloy composition
caused by the volatilization of the elements having the high vapor
pressure is prevented and the hydrogen-absorbing alloy having been
rapidly quenched has a target composition.
The thus obtained molten alloy is injected onto the traveling
surface (cooling surface) of the cooling roll(s) at a predetermined
pressure and rapidly quenched and solidified to be made to the
ribbon-shaped or flake-shaped hydrogen-absorbing alloy. At this
time, columnar structure are grown from the surface of the cooling
roll(s) toward the high temperature portion of the molten alloy,
that is, in the vertical direction with respect the surface of the
cooling roll(s). To enable the columnar structure to be
sufficiently grown, the thickness of the ribbon-shaped or
flake-shaped hydrogen-absorbing alloy is set to the range of 10-150
microns and preferably to the range of 15-100 microns and the
peripheral speed of the traveling surface of the cooling roll(s) is
set to the range of 5-15 m/sec. in the case of a copper roll and to
the range of 8-30 m/sec. in the case of an iron roll. When the
peripheral speed of the cooling roll(s) is less than the lower
limit of the above range, the molten alloy is quenched at a low
quenching rate and a columnar structure cannot be sufficiently
developed in the range of the above thickness. On the other hand,
when the peripheral speed of the cooling roll(s) exceeds the upper
limit of the above range, the molten alloy is driven off from the
cooling roll(s) at the moment it comes into contact therewith, and
thus the molten metal is not sufficiently quenched by the cooling
roll(s) so that the ratio of equi-axed crystals is increased. As a
result, the columnar structure having the area ratio of 50% in the
thickness direction thereof cannot be obtained, any way.
Further, the quenching rate for rapidly quenching the molten alloy
is preferably 1800.degree. C./sec. or higher as described above.
When the quenching rate is less than 1800.degree. C./sec., the
formation of the columnar structure with the special shape as
described above is impossible. The quenching rate is preferably set
to 2000.degree. C./sec. or higher and more preferably to
2400.degree. C./sec. or higher.
The above rapidly-quenching process of the molten metal is
preferably executed in an inert gas atmosphere of Ar or He and in
particular in vacuum to prevent the deterioration of the molten
alloy by oxidation. That is, when the rapidly-quenching process is
executed in the inert gas atmosphere, an inert gas may be rolled in
between the cooling roll(s) and the molten alloy, and thus the
conditions for achieving a sufficient quenching effect is narrowed.
On the other hand, when the processing is executed in vacuum, the
inert gas is not rolled in on the contrary to the case described
above, and thus the molten metal is sufficiently quenched on the
surface of the cooling roll(s).
Further, a material such as Cu group alloy, Fe group alloy, Ni
group alloy or the like excellent in thermal conductivity is used
as the material constituting the cooling roll. Further, a cooling
roll composed of the above material and having a surface hardened
by the formation of Cr plating or the like may be used.
Further, quartz generally used conventionally may be used as the
material for constituting the crucible for preparing the molten
alloy. A quartz crucible, however, has a drawback in that since it
does not produce heat when heated by high frequency induction
heating, the molten metal is cooled by the quartz when passing
through the outlet of the crucible and thus the outlet is liable to
be closed. The closing of the outlet can be effectively prevented
by using a crucible formed of ceramics of such as Ti-boride or the
like excellent in heat conductivity.
Since the hydrogen-absorbing alloy prepared by the above
molten-metal-rapidly-quenching method has a fine crystal structure
and a less amount of segregations of Mn or the like, when a
negative electrode is formed of it, the electrode capacity may be
improved to the level of 240 mAh/g or more and cycle life
characteristics to the level of 500 cycles or more at the same
time. Further, the inventors have found from the experiment that a
hydrogen-absorbing alloy for battery having more excellent
characteristics can be obtained by removing the internal distortion
of the alloy produced by the rapidly-quenching processing, by
paying attention to the internal distortion.
Thus, in the present invention, the above internal distortion is
removed by subjecting the hydrogen-absorbing alloy with the above
composition prepared by the molten-alloy-rapidly quenching
processing to a heat treatment at a relatively low temperature of
200.degree.-500.degree. C. for one hour or longer. More
specifically, as a result of the further studies of the inventors,
a new knowledge has been obtained that the capacity and life of the
hydrogen-absorbing alloy of the present invention prepared at the
aforesaid quenching rate of 1800.degree. C./sec. are further
improved together by being subjected to the heat treatment.
When the heat treatment temperature is less than 200.degree. C.,
the internal distortion is difficult to be removed, whereas when it
exceeds 500.degree. C., the composition of the alloy is changed by
the evaporation of the alloy components such as Mn and the like or
an alloy strength is lowered by a secondary recrystallization.
Therefore, the heat treatment temperature is set to the range of
200.degree.-500.degree. C. In particular, the range of
250.degree.-350.degree. C. is preferable to improve electrode
characteristics.
Further, when the heat treatment time is shorter than one hour, the
effect for removing the internal distortion is reduced. On the
other hand, when the heat treatment is executed for a longer time,
there is a possibility that the size of crystal particles is
increased and thus the heat treatment time is preferably about 2-5
hours by taking a manufacturing effect into consideration.
Note, the heat treatment atmosphere is preferably composed of an
inert gas or vacuum to prevent the oxidation of the
hydrogen-absorbing alloy at a high temperature.
As described above, the internal distortion of the
hydrogen-absorbing alloy can be effectively removed by the heat
treatment of the alloy effected at the relatively low temperature
while keeping the homogeneity thereof, and thus the electrode
capacity and life can be further extended. In particular, although
the effect of the heat treatment is low in a rapidly-quenched
molten alloy having a composition not containing Mn, when the
rapidly-quenched molten alloy of the present invention having the
composition containing Mn is subjected to the heat treatment, the
electrode capacity and battery life thereof can be greatly improved
together.
Next, a fourth hydrogen-absorbing alloy of the present invention
for achieving the above second object will be described.
At least 90 wt % of the fourth hydrogen-absorbing alloy of the
present invention is composed of AB.sub.x of single phase [where, A
is at least one kind of element selected from rare earth elements
including Y (yttrium), B is a metal mainly composed of Ni and at
least one kind of element selected from Co, Al, Fe, Si, Cr, Cu, Mn,
Ti, Zr, Zn, Hf, V, Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge and Sn,
5.05.ltoreq.x.ltoreq.6].
At least 90% of the fourth hydrogen-absorbing alloy for battery of
the present invention must have the single phase composed of the
AB.sub.x. If a phase (for example, phases composed of B, AB,
AB.sub.2, AB.sub.3, A.sub.2 B.sub.7, AB.sub.5, AB.sub.1.4 and the
like [hereinafter referred to a second phase]) other than the above
phase composed of AB.sub.x exceeds 10 wt % in the alloy, there are
increased chances in which two or more kinds of alloy phases having
a different composition come into contact with each other. The
boundaries of the alloy phases having the different composition
have a weak mechanical strength and cracks are liable to be formed
from these boundaries as hydrogen is absorbed and released.
Further, segregations are liable to be produced to the boundaries
and the alloy is liable to be corroded from the segregations.
Further, the second phase absorbs a less amount of hydrogen as
compared with AB.sub.x when used as an electrode, and thus when an
alloy including the second phase in an amount exceeding 10 wt % is
used as electrode, the electrode capacity per a unit volume is
reduced.
Further, the reason why the value of x is limited is as follows.
When x is less than 5.05, a hydrogen-absorbing alloy which is less
corroded when battery is charged/discharged and difficult to be
cracked and pulverized cannot be obtained. On the other hand, when
x exceeds 6, the creation of the second phase is admitted in the
alloy making method which can be usually employed in the industry
and thus the characteristics of the hydrogen-absorbing alloy cannot
be improved.
A constituting the AB.sub.x (5.05.ltoreq.x.ltoreq.6) shows at least
one kind selected from rare earth elements including Y
(specifically, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Lu).
Further, the element B shows a metal mainly composed of Ni and at
least one kind selected from Co, Al, Fe, Si, Cr, Cu, Mn, Ti, Zr,
Zn, Hf, V, Nb, Ta, Mo, W, Ag, Pd, B, Ga, In, Ge and Sn. In the
present invention, so long as the crystal system of the alloy
maintains a CaCu.sub.5 system, the type of crystals is not a
problem, but it preferably forms columnar structure.
When a hydrogen-absorbing alloy for battery is made from a molten
alloy having the composition exhibiting AB.sub.x by the method of
manufacturing the hydrogen-absorbing alloy according to the present
invention by using various kinds of molten-metal-rapidly-quenching
methods, the aforesaid hydrogen-absorbing alloy can be stably
obtained, and thus this manufacturing method is suitable. The
molten-metal-rapidly-quenching method includes the above rotating
disk method, single roll method, double roll method, gas atomizing
method and the like.
Next, a secondary nickel-metal hydride battery (cylindrical
secondary nickel-metal hydride battery) according to the present
invention using the above first to fourth hydrogen-absorbing alloys
as a negative electrode activating material will be described below
with reference to FIG. 3.
A hydrogen-absorbing alloy electrode (negative electrode) 11
containing the hydrogen-absorbing alloy is wound with a
non-sintering type nickel electrode (positive electrode) 12 to a
spiral-shape with a separator 13 disposed therebetween and
contained in cylindrical container 14 having a bottom. An alkaline
battery electrolyte is contained in the container 14.
A disc-shaped opening seal plate 16 having a hole 15 defined at the
center thereof is disposed on the upper opening of the container
14. A ring-shaped insulating gasket 17 is interposed between the
peripheral edge of the seal plate 16 and the inner surface of the
upper opening of the container 14 to fix the opening seal plate 16
to the container 14 in a gas-tight state through the gasket 17 by
narrowing the diameter of the above upper opening inwardly by
caulking. A positive electrode lead 18 has an end connected to the
positive electrode 12 and the other end connected to the lower
surface of the opening seal plate 16. A hat-shaped positive
electrode terminal 19 is mounted on the opening seal plate 16 to
cover the hole 15. A rubber safety valve 20 is disposed in the
space surrounded by the opening seal plate 16 and positive
electrode terminal 19 to close the hole 15. An insulating tube 21
is attached to the vicinity of the upper end of the container 14 to
fix the positive electrode terminal 19 and a collar 22 disposed on
the upper end of the container 14.
The above hydrogen-absorbing alloy electrode 11 includes a paste
type and a non-paste type as described below:
(1) a paste type hydrogen-absorbing alloy electrode is made in such
a manner that a hydrogen-absorbing alloy powder obtained by
pulverizing the above hydrogen-absorbing alloy, a polymer binder
and an electric conductive powder added when necessary are mixed to
make a paste and the paste is coated to and filled with an electric
conductive substrate as a collector and dried and then pressed by a
roller press or the like; and
(2) a non-paste type hydrogen-absorbing alloy electrode is made in
such a manner that the hydrogen-absorbing alloy powder, polymer
binder and electric conductive powder added when necessary are
stirred and dispersed to the electric conductive substrate as a
collector and then pressed by a roller press or the like.
As a method of pulverizing the hydrogen-absorbing alloy, there are
employed a mechanical pulverizing method effected by a ball mill,
pulverizer, jet mill or the like and a method of causing the
hydrogen-absorbing alloy to absorb/release high pressure hydrogen
and pulverizing the same by the expansion of the volume thereof at
the time.
The polymer binder includes, for example, sodium polyacrylate,
polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC),
polyvinyl alcohol (PVA) and the like. Each of these polymer binders
in the range of 0.5-5 weight parts is preferably combined with 100
weight parts of the hydrogen-absorbing alloy. When, however, the
non-paste type hydrogen-absorbing alloy electrode of the above item
(2) is to be made, polytetrafluoroethylene (PTFE) is preferably
used as the polymer binder because PTFE is made to fibers by being
stirred and can fix the hydrogen-absorbing alloy powder and the
electric conductive powder added when necessary to a
three-dimensional state (mesh-state).
The electric conductive powder includes, for example, a carbon
powder such as a graphite powder, ketchen black and the like and a
metal powder such as a nickel powder, copper powder, cobalt powder
and the like. Each of these electric conductive powders in the
range of 0.1-5 weight parts is preferably combined with 100 weight
parts of the hydrogen-absorbing alloy.
The electric conductive substrate includes, for example, a
two-dimensional substrate such as a punched metal, expanded metal,
wire net and the like and a three-dimensional substrate such as a
foamed metal substrate, net-shaped sintered fiber substrate,
plated-felt substrate composed of a non-woven fabric to which metal
is plated, and the like. When, however, the non-paste type
hydrogen-absorbing alloy electrode of the above item (2) is made,
the two-dimensional substrate is preferably used as the electric
conductive substrate because a combined material containing the
hydrogen-absorbing alloy powder is dispersed.
The non-sintering type nickel electrode 12 combined with the
hydrogen-absorbing alloy electrode is made in such a manner that a
paste is prepared by suitably combining the mixture of nickel
hydroxide, cobalt hydroxide (Co(OH).sub.2) added when necessary and
cobalt monoxide (CoO), metallic cobalt and the like with
polyacrylate such as carboxymethyl cellulose (CMC), sodium
polyacrylate and the like, and the paste is filled with the
three-dimensional substrate such as the foamed metal substrate,
net-shaped sintered fiber substrate, plated-felt substrate composed
of the non-woven fabric to which metal is plated, and the like and
dried and then pressed by the roller press or the like.
A non-woven fabric composed of polymer fibers used as the separator
13 includes simple polymer fibers such as nylon, polypropylene,
polyethylene and the like and complex polymer fibers mixed with
these polymer fibers.
Used as the alkali battery electrolyte is, for example, a 6N to 9N
potassium hydroxide solution or the potassium hydroxide solution
mixed with lithium hydroxide, sodium hydroxide or the like.
Next, a nickel-metal hydride battery achieving the third object of
the present invention will be described.
In the secondary nickel-metal hydride battery according to the
present invention, an alloy made by a
molten-metal-rapidly-quenching method is used a hydrogen-absorbing
alloy constituting a hydrogen-absorbing alloy electrode. Further,
the amount of the battery electrolyte of the secondary nickel-metal
hydride battery composed by using the hydrogen-absorbing alloy
electrode is within the range of 0.4-1.8 ml/Ah with respect to the
capacity of the hydrogen-absorbing alloy electrode and further the
capacity ratio of an alloy in an uncharged state in the battery
discharged state of the hydrogen-absorbing electrode to the
capacity of a nickel electrode is set within the range of 1.1-2.0.
The hydrogen-absorbing alloy is made by the aforesaid
molten-metal-rapidly-quenching method such as the rotating disc
method, single roll method, double roll method, gas atomizing
method and the like.
The reason why the amount of the battery electrolyte is limited to
the above range in the relationship with the hydrogen-absorbing
alloy electrode is as follows. When the amount of the battery
electrolyte is less than 0.4 ml/Ah, the area in which the
hydrogen-absorbing alloy electrode is in contact with the battery
electrolyte is reduced, and thus a smooth battery reaction is
prevented and the capacity of the nickel electrode constituting the
battery cannot be sufficiently extracted, with the result of the
reduction of the battery capacity. When the amount of the battery
electrolyte exceeds 1.8 ml/Ah, the surface of the
hydrogen-absorbing alloy electrode is covered with an excessive
battery electrolyte and thus the smooth reduction of the oxygen gas
evolved from the nickel electrode is prevented in an excessively
charged state. As a result, a battery inner pressure is increased
and battery characteristics are deteriorated together with the
operation of the safety valve.
The reason why the capacity ratio of an alloy in an uncharged state
in the battery discharged state of the hydrogen-absorbing electrode
to the capacity of a nickel electrode is set within the above range
is as follows. When the capacity ratio is less than 1.1, since the
battery capacity is limited by the capacity of the
hydrogen-absorbing alloy electrode, the battery inner pressure is
increased at the end of charge/discharge operation unless a special
charge/discharge method is employed, and thus the battery
characteristics are deteriorated together with the operation of the
safety valve. On the other hand, when the capacity ratio exceeds
2.0, the battery internal pressure is slightly increased, because
the volume of the vacant space in the battery is reduced as well as
the volume occupied by the hydrogen-absorbing electrode is
increased in the battery container. As a result, the amount of the
nickel electrode as a volume-limited electrode is reduced with
reduction of the battery capacity.
The first to third hydrogen-absorbing alloys composed as described
above are an AB.sub.5 type alloy containing Mn as an essential
element and excellent in the cycle life and initial characteristics
with a high capacity, and further can provide the negative
electrode material with a stable charging potential for
nickel-hydrogen battery. These alloys can be obtained by rapidly
quenching a molten alloy having a predetermined composition at a
quenching rate of 1800.degree. C./sec. or higher.
In addition, when thus prepared alloy is further subjected to a
heat treatment at a relatively low temperature of about
200.degree.-500.degree. C., the internal distortion of the alloy
can be effectively eliminated while keeping the homogeneity of the
alloy, whereby there can also be provided a further improved
nickel-metal hydride battery excellent in battery
characteristics.
Although the reason why the first hydrogen-absorbing alloy with the
above crystal structure has the aforesaid excellent characteristics
is not clear, it is supposed to be resulted from the following
operation. That is, in the columnar structure in which the crystals
of the hydrogen-absorbing alloy are grown in the same direction,
the alloy is expanded and shrank in a given direction as it absorbs
and releases hydrogen, and thus the pulverization of the alloy is
suppressed by the reduction of the stress in the alloy. As a
result, since the increase of the area of the alloy in contact with
the battery electrolyte is suppressed, the corrosion of the alloy
is prevented. Further, in the hydrogen-absorbing alloy made by the
conventional casting method, segregations are dispersed to particle
boundaries from which corrosion begins.
It is supposed, however, that in the columnar structure having
crystals grown from the quenched surface in the same direction as
in the case of the present invention, the segregations of elements
are concentrated to the particular locations of the alloy as the
crystals are grown, and thus the locations from which corrosion
begins are reduced. Therefore, the cycle characteristics of the
secondly nickel-metal hydride battery to which the electrode
containing the hydrogen-absorbing alloy is assembled are greatly
improved.
The dispersion of a Mn concentration with respect to the second
hydrogen-absorbing alloy according to the present invention will be
described. An alloy, in which the maximum value of the Mn
concentrations distributed therein exhibits a value exceeding 1.3
times the average value of the Mn concentrations in the entire
alloy, has the locations in which partially large Mn concentrations
are dispersed in the alloy. When the alloy is used to the negative
electrode of the nickel-metal hydride battery, a corrosion reaction
is liable to be caused in these locations and the thus the
deterioration of the hydrogen-absorbing alloy itself is liable to
be progressed.
Further, since the degree of expansion/shrinkage of the
hydrogen-absorbing alloy volume caused by the hydrogen
absorption/release due to an electrode reaction is made partially
different by the existence of the locations having the different Mn
concentrations, stress is produced in the alloy and pulverization
is liable to be progressed to thereby increase a specific surface
area, and thus the deterioration of the hydrogen-absorbing alloy is
further accelerated.
The above phenomenon is difficult to be caused in an alloy in which
the maximum value of the Mn concentrations distributed therein
exhibits a value of 1.3 times or less the average value of the Mn
concentrations in the entire alloy. Therefore, it is supposed that
when this alloy is applied to a negative electrode, the progress of
corrosion of the alloy is suppressed so that the cycle life
characteristics of the nickel-metal hydride battery are
improved.
Next, the maximum diameter of Mn segregations according to the
third hydrogen-absorbing alloy of the present invention will be
described. When an alloy, in which the maximum diameter of Mn
segregations exceeds 0.5 micron, is used to the nickel-metal
hydride battery, a corrosive reaction begins from the segregations
because a segregated portion has a large size, and thus the
deterioration of the hydrogen-absorbing alloy itself is liable to
be progressed. Further, when the alloy is expanded/shrank by the
hydrogen absorption/release due to an electrode reaction, since
stress is concentrated to the segregated points of Mn, cracks are
liable to be made from the segregated points to thereby further
accelerate pulverization.
On the other hand, the above phenomenon is difficult to be caused
in a hydrogen-absorbing alloy in which the maximum diameter of Mn
segregations is 0.5 micron or less. Therefore, it is supposed that
when this alloy is applied to the negative electrode of a
nickel-metal hydride battery, the progress of corrosion of the
negative electrode alloy is suppressed so that the cycle life
characteristics of the nickel-metal hydride battery are
improved.
Further, when the fourth hydrogen-absorbing alloy according to the
present invention at least 90 wt % of which is composed of AB.sub.x
of single phase [where, A is at least one kind of element selected
from rare earth elements including Y (yttrium), B is a metal mainly
composed of Ni and at least one kind of element selected from Co,
Al, Fe, Si, Cr, Cu, Mn, Ti, Zr, Zn, Hf, V, Nb, Ta, Mo, W, Ag, Pd,
B, Ga, In, Ge and Sn, 5.05.ltoreq.x.ltoreq.6] is used as an
activating material to form an electrode and a battery is arranged
by using the electrode, a corrosion resistance to a thick alkaline
battery electrolyte can be greatly improved. As a result, a
secondary alkaline battery (e.g., secondary nickel-metal hydride
battery) in which characteristics such as a cycle life and the like
are improved can be realized.
Although the reason why the hydrogen-absorbing alloy with the alloy
composition of the AB.sub.x of the single phase has the excellent
corrosion resistance is not clear, this is supposed to be resulted
from the following behavior.
That is, in a hydrogen-absorbing alloy whose crystal structure is
found not to include a CaCu.sub.5 type single phase but include a
plurality of phases in an amount of 10 wt % or more when observed
by an X-ray diffraction, there are increased chances in which alloy
phases having a different composition come into contact with each
other. The boundaries between the alloy phases having the different
composition have a weak mechanical strength and cracks are liable
to be made from the boundaries as hydrogen is absorbed/released.
Further, corrosion is liable to be made to the boundaries due to
segregations. Further, corrosion is liable to be made to the
boundaries by the segregations. In addition, the phases having a
composition other than AB.sub.x absorb a less amount of hydrogen
ions as compared with the phase having AB.sub.x when used as an
electrode. As a result, when the secondary nickel-metal hydride
battery to which the electrode containing the hydrogen-absorbing
alloy as the activating material is assembled is evaluated, not
only the improvement of life is not admitted but also the reduction
of capacity is admitted.
From the mentioned above, when at least 90 wt % of the
hydrogen-absorbing alloy is composed of the AB.sub.x of the single
phase, cracks and corrosion can be suppressed and life can improved
when the alloy is used as electrode. Further, since a
hydrogen-absorbing alloy having a non-stoichiometric composition,
although the crystal structure thereof is the CaCu.sub.5 type,
makes distortion in crystals, the alloy endures the
expansion/shrinkage due to hydrogen absorption/release and as a
result it can extend a cycle life of the electrode. Further,
hydrogen is smoothly dispersed. As a result, when this
hydrogen-absorbing alloy is assembled to battery, it has an effect
to improve large current discharge characteristics.
On the other hand, the hydrogen-absorbing alloy made by the
molten-metal-rapidly-quenching method can homogenize a composition
as well as pulverize a crystal size and further suppress
solidification and segregation when the alloy is quenched as
compared with the hydrogen-absorbing alloy made by the conventional
method of melting a material in a crucible in an inert gas
atmosphere or vacuum and then casing the same to a casting mold. As
a result, when an electrode is formed by using the
hydrogen-absorbing alloy as an activating material and a battery is
arranged by using the electrode, a speed of corrosion caused by an
alkaline battery electrolyte can be reduced.
When the secondary nickel-metal hydride battery is arranged by
using the negative electrode containing the hydrogen-absorbing
alloy made by the molten-metal-rapidly-quenching method, however,
there is a case wherein when a battery design similar to that
applied to the alloy made by the conventional method is used, not
only the feature of the hydrogen-absorbing alloy made by the
molten-metal-rapidly-quenching method cannot not be exhibited but
also particularly when the amount of a battery electrolyte is
excessively large, even the battery characteristics of the alloy
made by the conventional method cannot be reproduced.
From the mentioned above, the secondary nickel-metal hydride
battery according to the present invention can achieve a smooth
battery reaction and sufficiently extract the capacity of a nickel
electrode to thereby improve a capacity by limiting the amount of
the battery electrolyte within the range of 0.4-1.8 ml/Ah to the
capacity of a hydrogen-absorbing alloy negative electrode.
Furthermore, oxygen gas produced from the nickel electrode in an
excessively charged state can be smoothly reduced, whereby the
increase of the battery internal pressure can be suppressed.
Further, the battery capacity is avoided from being limited by the
capacity of the hydrogen-absorbing alloy electrode in such a manner
that the capacity ratio of an alloy in an uncharged state in the
battery discharged state of the hydrogen-absorbing electrode to the
capacity of a nickel electrode is set within the range of 1.1-2.0
and the increase of the battery internal pressure at the ends of
charge and discharge can be restricted without the need of using a
special charge/discharge method. Simultaneously, the volume
occupied by the hydrogen-absorbing alloy electrode in the battery
container can be reduced. Corresponding to the reduced volume, a
volume of the nickel electrode as a volume-limited electrode can be
increased, thereby to increase the battery capacity.
Accordingly, there can be provided a secondary nickel-metal hydride
battery having a high capacity and a long cycle life at a low cost,
the battery utilizing the characteristics of the hydrogen-absorbing
alloy manufactured by molten-metal-rapidly-quenching method, and
enabling to suppress the increase of the battery internal
pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing a
molten-metal-rapidly-quenching apparatus using a single roll
method;
FIG. 2 is a schematic diagram showing a
molten-molten-metal-rapidly-quenching apparatus using a double roll
method;
FIG. 3 is a perspective view showing an example of the arrangement
of a nickel-metal hydride battery according to the present
invention;
FIG. 4 is a photograph taken by an electron microscope to show the
metal structure of a flake-shaped hydrogen-absorbing alloy
subjected to a molten-molten-metal-rapidly-quenching
processing;
FIG. 5 is a photograph taken by an electron microscope to show the
metal structure of hydrogen-absorbing alloy particles contained in
a negative electrode taken out from a battery;
FIG. 6 is a photograph taken by an electron microscope to show the
metal structure of hydrogen-absorbing alloy particles contained in
the negative electrode taken out from the battery;
FIG. 7 is a photograph taken by an electron microscope to show the
metal structure of hydrogen-absorbing alloy particles contained in
the negative electrode taken out from the battery;
FIG. 8 is a photograph taken by an electron microscope to show the
metal structure of hydrogen-absorbing alloy particles contained in
the negative electrode taken out from the battery;
FIG. 9 is a photograph taken by an electron microscope to show the
metal structure of the hydrogen-absorbing alloy according to an
embodiment 9B;
FIG. 10 is a schematic diagram copying the columnar structure of
the metal structure shown in FIG. 9;
FIG. 11 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:5 or higher in the metal structure
shown in FIG. 9;
FIG. 12 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:4 or higher in the metal structure
shown in FIG. 9;
FIG. 13 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:3 or higher in the metal structure
shown in FIG. 9;
FIG. 14 is a schematic diagram showing the columnar structure
having an aspect ration of 1:2 or higher in the metal structure
shown in FIG. 9;
FIG. 15 is a photograph taken by an electron microscope to show the
metal structure of the hydrogen-absorbing alloy according to an
embodiment 5A;
FIG. 16 is a schematic diagram copying the columnar structure of
the metal structure shown in FIG. 15;
FIG. 17 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:5 or higher in the metal structure
shown in FIG. 15;
FIG. 18 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:4 or higher in the metal structure
shown in FIG. 15;
FIG. 19 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:3 or higher in the metal structure
shown in FIG. 15;
FIG. 20 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:2 or higher in the metal structure
shown in FIG. 15;
FIG. 21 is a photograph taken by an electron microscope to show the
metal structure of the hydrogen-absorbing alloy according to an
embodiment 12A;
FIG. 22 is a schematic diagram copying the columnar structure of
the metal structure shown in FIG. 21;
FIG. 23 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:5 or higher in the metal structure
shown in FIG. 21;
FIG. 24 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:4 or higher in the metal structure
shown in FIG. 21;
FIG. 25 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:3 or higher in the metal structure
shown in FIG. 21;
FIG. 26 is a schematic diagram showing the columnar structure
having an aspect ratio of 1:2 or higher in the metal structure
shown in FIG. 21;
FIG. 27 is a photograph taken by an electron microscope to show the
metal structure of the hydrogen-absorbing alloy according to a
comparative example 7B;
FIG. 28 is a characteristic diagram showing the relationship
between the diameter of crystal particles of a hydrogen absorbing
alloy and the cycle life of the battery;
FIG. 29 is a characteristic diagram showing the relationship
between the ratio of the maximum value to the average value of the
Mn concentration in an alloy and the cycle life of the battery;
FIG. 30 is a characteristic diagram showing the relationship
between the maximum diameter of the Mn particles segregated in an
alloy and the cycle life of the battery;
FIG. 31 is a characteristic diagram showing the relationship
between the composition ratio x, electrode capacity, and the cycle
life in an embodiment 19;
FIG. 32 is a schematic cross sectional view showing an apparatus
for making hydrogen absorbing-alloy particles by a rotating disc
method;
FIG. 33 is a schematic cross sectional view showing an apparatus
for making hydrogen absorbing-alloy particles by a gas atomizing
method;
FIG. 34 is a graph comparing cycle life of a battery resulting from
various capacity ratios of negative and positive electrode alloys
and various amounts of a battery electrolyte;
FIG. 35 is a graph comparing the maximum values of a battery
internal pressure resulting from various capacity ratios of
negative and positive electrode alloys and various amounts of a
battery electrolyte;
FIG. 36 is a graph comparing maximum battery capacities resulting
from various capacity ratios of negative and positive electrode
alloys and various amounts of a battery electrolyte;
FIG. 37 is a graph showing the relationship between the
charge/discharge cycle and the battery capacity of comparative
examples 25, 26 and 27 and examples 22, 23 and 24.
FIG. 38 is a graph showing the relationship between the
charge/discharge cycle and the battery capacity of an AA type
battery.
FIG. 39 is a graph showing the relationship between the
charge/discharge cycle and the battery internal pressure of the AA
type battery.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be described below more
specifically.
Embodiments 1-9
Mixture of various law materials were prepared, taking the amount
of the law materials lost when they were melted into consideration,
so that rapidly-quenched molten alloys obtained by being
rapidly-quenched from the melt had the compositions shown in Table
1. The mixtures of the various law materials were put into a
crucible made of Ti-boride and melted by a high frequency induction
heating method, and various molten metal alloys were prepared.
Next, the thus obtained various molten metal alloys were injected
onto the surface of the cooling roll of an apparatus employing a
single roll method shown in FIG. 1 so that flake-shaped
rapidly-quenched molten alloys having a thickness of 50 microns
were prepared. The cooling roll was composed of a copper roll
having a diameter of 300 mm and the distance between an injection
nozzle and the cooling roll was set to 50 mm and an injection
pressure was set to 0.02 Kgf/cm.sup.2.
Further, the rapid quenching was executed in vacuum, the rotating
speed (rpm) of the cooling roll was set to 600 rpm and an
adjustment was performed so that the molten metal alloy had an
average quenching rate of 2400.degree. C./sec.
Next, the thus obtained respective rapidly-quenched molten alloys
were pulverized and classified to 200 mesh or less so that hydrogen
absorbing-alloy powders for battery was prepared. Next, the thus
prepared hydrogen absorbing-alloy powders for battery, PTFE powder
and carbon powder were weighed so that they were 95.5 wt %, 4.0 wt
% and 0.5 wt %, respectively and then kneaded so that respective
electrode sheets were made.
The electrode sheets were cut off to a predetermined size and
attached under pressure to a nickel collector to make hydrogen
absorbing-alloy electrodes, respectively.
Then, the respective hydrogen absorbing-alloy electrodes were
charged up to 300 mAh/g with a current (220 mA/g) per 1 g of alloy
and discharged with the above current until a potential difference
of -0.5 V was achieved with respect to an Hg/HgO reference
electrode and at this time a maximum electrode capacities were
measured. The result of the measurements is shown Table 1. Further,
the number of activations of the respective electrodes was
measured. The number of activations means the number of
charge/discharge cycles necessary for the electrodes thus made to
exhibit a maximum capacity and serves as an index for determining
the good or bad rising-up of the battery characteristics of
batteries made by using these alloys.
On the other hand, a paste was prepared by adding a small amount of
CMC (carboxymethyl cellulose) and water to 90 wt % of nickel
hydroxide and 10 wt % of cobalt monoxide and stirring and mixing
them. This paste was filled to a porous nickel member having a
three-dimensional structure and dried and rolled by a roller press
to make a nickel electrode.
Then, AA type nickel-metal hydride batteries of the respective
embodiments were assembled by combining the above respective
hydrogen absorbing-alloy electrodes and the nickel electrode. Here,
the capacity of the respective batteries was set to 650 mAh as the
theoretical capacity of the nickel electrode and a solution mixed
with 7N potassium hydroxide and 1N lithium hydroxide was used as a
battery electrolyte.
Next, the respective batteries were charged with 650 mA for 1.5
hours and repeatedly subjected to charge/discharge cycles so that a
current was discharged at the current of 1A until the batteries had
a voltage of 1 V and the number of the cycles at which the capacity
of the batteries became 80% of an initial capacity was measured as
a battery life.
Further, the respective batteries were assembled and charged 10
cycles and then they were disassembled to analyze the metal
structure of the hydrogen absorbing-alloys used as the negative
electrodes. Note, the hydrogen absorbing-alloy powder assembled
once into the battery was integrated with a polymer binder and the
like and further partially corroded by the electrolyte and an
etching solution used to observe a fine structure, and thus it was
difficult to observe the metal structure thereof as it was. Thus,
specimens for analysis were made by the procedures shown in the
following items (1)-(3) and further the metal structure of the
specimens was analyzed by the method shown in item (4).
(1) Taking-Out of Negative Electrode
When the battery is disassembled in the state that it is not
perfectly discharged, there is a possibility that the hydrogen
absorbing-alloy contained in the negative electrode may be fired,
and thus the nickel-metal hydride battery is perfectly discharged
and then the battery is disassembled and the negative electrode is
taken out. To prevent the firing, the negative electrode taken out
from the battery is sufficiently washed with water and dried. If
the negative electrode is not sufficiently washed with water and
dried, it is not in intimate contact with a resin in the next resin
burying process and may be exfoliated from the resin.
(2) Burying of Negative Electrode by Resin
Ten pieces of specimens of 10 mm.times.5 mm are cut off from the
dried negative electrode and numbers from 1 to 10 were put to the
specimens. Each of these specimens is vertically disposed in a mold
so that longer side thereof is directed downward and a gap
therebetween was filled with a resin flowed into it to form a
composite member. A resin having a low viscosity such as an epoxy
resin is used as the resin to be buried. Further, it is preferable
that the resin is flowed into the mold in the state that the
temperature thereof is increased to lower its viscosity so that the
specimen is in intimate contact with the resin.
(3) Polishing of Composite Member
The surface of the cured composite member is polished by
sequentially using water resistant abrasive papers (#600)-(#1500)
to expose the cross section of the hydrogen absorbing alloy. This
polishing operation may be carried out by a polishing machine. In
this case, however, since the hydrogen absorbing-alloy in the
electrode is liable to be exfoliated due to an excessive impact
force, it is preferable that the polishing operation is manually
carried out.
(4) Observation of Alloy Structure
When the metal structure of the hydrogen absorbing alloy contained
in the specimens is observed by a scanning type electron microscope
(SEM), the metal structure may not be partially confirmed clearly
because the hydrogen absorbing alloy is integrated with the polymer
binder. Therefore, only the alloys whose metal structure can be
clearly recognized are selected as the objects of the
observation.
The observation was carried out by selecting a visual field in
which the metal structure of the hydrogen-absorbing alloy exposed
to the cross section of the electrode could be observed as much as
possible within the region R partitioned by a maximum rectangular
shape in which the crystal structure of the alloy integrally
touched when the metal structure is photographed by using the SEM
at a magnification of, for example, 1500-2000 as shown in FIGS. 10,
16 and 22, the above crystal structure being a portion of the
entire indefinite crystal structure in which crystal particles
could be visually confirmed. At that time, with respect to each of
the hydrogen-absorbing alloys of the present invention contained in
the negative electrode, the ratio of the crystal particles having
the aspect ratio of 1:2 or higher occupying in the metal structures
which can be recognized is preferably 50% or higher, more
preferably 70% or higher and further more preferably 80% or higher.
This is because that when the ratio is less than 50%, a crystal
particle boundaries are increased and as a result segregations are
also increased, and thus the alloy particles are greatly
deteriorated. Further, it is necessary that the ratio of the number
of the alloy particles with crystal particles having the aspect
ratio of 1:2 or higher and the area ratio of the columnar of 50% or
more is preferably 30% or more, more preferably 50% or more and
further more preferably 70% or more with respect to the number of
the entire particles of the hydrogen-absorbing alloy having the
metal structures which can be visually recognized. When the ratio
of the number of the crystal particles is less than 30%, the number
of the crystal particles liable to be deteriorated is relatively
increased, and when a battery is formed, the life thereof is
remarkably shortened.
The aspect ratio is the ratio of the minor diameter of a crystal
particle to the major diameter thereof, and the major diameter is
defined as the maximum length in the axial direction of the crystal
particle and the minor diameter as the maximum length in the
direction perpendicular to the axis.
In the metal structure of the hydrogen-absorbing alloy according to
the present invention, the columnar crystal particles preferably
have an average minor diameter of 30 microns or less, more
preferably 20 microns or less and further more preferably 10
microns or less. When the average minor diameter exceeds 30
microns, many segregations are produced and the corrosion
resistance of the alloy is lowered and the life thereof is
shortened as well as a charge/discharge cycle is greatly lowered
when a battery is made. Further, since the area of the crystal
boundaries as the moving paths of hydrogen in the alloy is reduced,
the hydrogen has an increased diffusion resistance and thus a
battery voltage is liable to be lowered when a large current
discharges.
Next, a method of determining the aspect ratio of the columnar
structure constituting the metal structure of the
hydrogen-absorbing alloy will be specifically described.
FIG. 4 is a photograph taken by an electron microscope to show an
example of the metal structure of the hydrogen-absorbing alloy
according to this embodiment and the photograph shows the cross
section in the thickness direction of a rapidly-quenched molten
alloy in contact with the cooling roll of the single roll
apparatus. The hydrogen-absorbing alloy is a flake-shaped
rapidly-quenched molten alloy before it is pulverized. In FIG. 4,
the lower side of the cross section is a quenched surface in
contact with the cooling roll and the upper side thereof is a free
side. It can be clearly recognized the state that columnar
structure having various aspect ratios is vertically grown from the
quenched surface by the molten-alloy-rapidly-quenching process.
However, the alloy obtained by pulverizing the flake-shaped
rapidly-quenched molten alloy is used to the negative electrode of
an actual battery and the pulverized powder of the alloy is
sometimes eroded by a battery electrolyte, and thus even if the
columnar structure is observed by the SEM, the columnar structure
is not always clearly observed. The cases in which the metal
structure is made unclear will be described below with reference to
FIGS. 5-8.
FIGS. 5-8 show the photographs taken by the SEM of the
hydrogen-absorbing alloy contained in the negative electrode taken
out from an actually used battery. More specifically, in FIG. 5, a
cross section in the vertical direction with respect to the
quenching surface of the cooling roll is observed and the state
that columnar structure is vertically grown can be recognized. FIG.
6 shows the surface structure of the free side opposite to the side
in contact with the cooling roll (the roll side). In this case,
since only the edge faces corresponding to the minor diameter of
columnar structure is observed, the columnar structure may be
observed as if they were equi-axed crystals. In FIG. 7, although
columnar structure is partially observed at the central portion
thereof, since the right upper portion thereof is eroded by a
battery electrolyte, it is difficult to clearly recognize a metal
structure as a whole. FIG. 8 shows an example in which the upper
surface (white portion) of a hydrogen-absorbing alloy is eroded by
a battery electrolyte when observed by the SEM and a metal
structure cannot be clearly recognized as a whole.
Thus, when the metal structure of the hydrogen-absorbing alloy
contained in a negative electrode is to be observed, an object
surface is mirror-polished and etched to expose the crystal
boundaries of the metal structure. When, however, an etching
solution is not suitable to the composition of the
hydrogen-absorbing alloy, the etching solution excessively erodes
the entire alloy, and thus the crystal boundaries cannot be clearly
exposed. Therefore, even if columnar structure is formed, when they
are greatly eroded, the recognition of the metal structure as a
whole is difficult to be recognized. On the other hand, even when
they are slightly eroded, crystal particle boundaries are difficult
to be clearly recognized although the configuration of the columnar
structure can be barely recognized visually. Note, in addition to
the above cases, when a cross section perpendicular to the cooling
roll is observed, columnar structure may observed as if they were
equi-axed crystals depending upon an angle between the surface of
the cooling roll and the cross section. Therefore, in this case, an
attention must be taken when the area ratio of the columnar
structure is measured.
FIGS. 9, 15 and 21 show the photographs taken by the SEM of the
hydrogen-absorbing alloy particles contained in the negative
electrode taken out from actually used batteries. The
hydrogen-absorbing alloy particles shown in FIGS. 9, 15 and 21
correspond to embodiments 5, 1 and 8, respectively.
Although clear alloy particles in which columnar structure is grown
are observed as shown in the central portion of FIG. 15, the clear
alloy particles are also surrounded by alloy particles whose
crystal structure cannot be clearly observed because they are
eroded by the electrolyte of a battery or an etching solution used
to observe them with the SEM.
In particular, when the etching solution is not suitable to the
composition of the hydrogen-absorbing alloy, the metal structure of
the alloy cannot be observed because the surface of the alloy is
eroded by the etching solution.
A method of calculating the ratio of crystal particle having the
aspect ratio of 1:2 or higher occupying in the entire crystal
particles based on the above SEM photographs will be described.
First, the number of hydrogen-absorbing alloy particles whose metal
structure can be clearly observed is counted in the entire alloy
particles observed in the cross section of a single polished piece
of a negative electrode and the count value is represented by N1.
At this time, a specific signal from an rare earth element is
detected by using an X-ray microanalyzer (EPMA), energy dispersion
type X-ray analyzer (EDX) or the like to confirm that the alloy
particles are composed of the hydrogen-absorbing alloy. When this
specific signal is not detected from the rare earth element, the
alloy particles are assumed to be stuck materials such as
pulverized pieces and determined not to be the hydrogen-absorbing
alloy particles and excluded from the number of the alloy
particles.
Next, the number of the alloy particles N1' in which crystal
particles having the aspect ratio of 1:2 or higher occupy 50% or
more of the area of the metal structure is counted in the N1 pieces
of the alloy particles. Hereinafter, the same count will executed
to the respective pieces of the specimens of the negative
electrode.
The ratio of the columnar structure is calculated by substituting
thus determined values of N1-N10 and N1'-N10' for the following
equation.
Next, a method of calculating the ratio of the columnar structure
occupying in the metal structure will be described. Note, the long
crystal particles having the aspect ratio of 1:2 were made to the
object to be investigated as the columnar structure. Further, not
only equi-axed crystals but also chill crystals and various stuck
materials were included in the crystal particles having the aspect
ratio of less than 1:2 . The area occupied by the columnar
structure was measured by using an image analyzer (Model LUZEX 500
made by Nippon Regulator Co., Ltd.). More specifically, when
description is made by using the embodiment 5 as an example, a thin
tracing paper (basis weight: about 40 g/m.sup.2) was placed on the
SEM photograph of the metal structure shown in FIG. 9 and particle
boundaries were copied to the tracing paper so that a copied paper
as shown in FIG. 10 was created. In the hydrogen-absorbing alloy
particles, an eroded portion 30 eroded by a battery electrolyte was
formed on the left side of columnar structure 31. Further, a stuck
material 33 such as a fragment or the like of a pulverized matter
was located at the central portion of the region R.
Next, the portion corresponding to the columnar structure having
the aspect ratio of 1:5 or higher was colored with black to obtain
FIG. 11. Then, the portion corresponding to the columnar structure
having the aspect ratio of 1:4 or higher was colored with black to
obtain FIG. 12 in the same way. Next, the portions having the
aspect ratios of 1:3 and 1:2 or higher were colored with black in
the same way to obtain FIGS. 13 and 14. Next, FIGS. 11-14 were
subjected to an image processing by using an image analyzer so that
the area ratios of the columnar structures corresponding to these
aspect ratios were optically analyzed and calculated. More
specifically, the image analyzer recognized the presence and
absence of the columnar structure in the object range R by a shade
of color and calculated the area ratios.
FIGS. 15-20 and FIGS. 21-26 show the examples in which the area
ratio is cumulatively calculated with respect to other alloy
structures. FIG. 15 is an SEM photograph showing the
hydrogen-absorbing alloy particles according to the embodiment 1.
The copy shown in FIG. 16 was obtained by tracing the crystal
structure of the alloy particles at the central portion which could
be clearly recognized in the alloy particles shown in the SEM
photograph of FIG. 15. In FIG. 16, the linear portion at the upper
edge is an abutted face 34 abutted against a cooling roll and fine
chill crystals 35 made by the super-rapid quenching action of the
cooling roll are located along the abutted face 34. Note, since the
chill crystals 35 are fine, the crystal boundaries thereof are not
shown. Further, a stuck material 33 is located on the right side of
the region R.
Next, the portions of the columnar structures having the aspect
ratios of 1:5 or higher, 1:4 or higher, 1:3 or higher and 1:2 or
higher are colored with black, respectively to obtain the copies
shown in FIGS. 17-20 in the same way as the embodiment 5.
The copy of FIG. 22 showing the crystal structure of the
hydrogen-absorbing alloy according to the embodiment 8 was made
from the SEM photograph shown in FIG. 21 in the same way and the
columnar structures corresponding to respective aspect ratios were
colored with black to make FIGS. 23-26 which were analyzed by the
image analyzer to determine the area ratios of the columnar
structure portions.
As apparent from FIGS. 9-26, it was confirmed that columnar
structures 31 were sufficiently grown in any of the crystal
structures, whereas equi-axed crystals 32 partially existed in the
structures.
Note, as shown in FIGS. 10, 16 and 22, the region subjected to the
above analysis was limited to the region R partitioned by a maximum
rectangular shape in which the crystal structure of the alloy
integrally touched in the entire indefinite crystal structure in
which crystal particles could be visually confirmed. Note, the
columnar structure located on the boundary of the region R employed
the area thereof located only in the region R and the aspect ratios
of them were assumed from the entire configuration of the columnar
structures including their portions located to the outside of the
region.
Table 1 shows the area ratio and minor diameter of the columnar
structure of the respective hydrogen-absorbing alloys according to
the embodiments 1-9, the maximum electrode capacity of the
electrodes and the activation number of the electrodes using the
hydrogen-absorbing alloys, and the number charge/discharge cycles
of the batteries.
TABLE 1
__________________________________________________________________________
Characteristics Area Ratio of Columnar Structures Minor Number RPM
of Aspect Ratio Dia of of Manu- Cooling 1:2 1:3 1:4 1:5 Columnar
Electrode Cycle Acti- Specimen facturing Roll or or or or As a
Structures Capacity Life vation No. Alloy Composition Method
(r.p.m) higher higher higher higher whole (.mu.m) (mAh/g) (cycles)
(times)
__________________________________________________________________________
Embodi- LmNi.sub.4.0 Co.sub.0.4 Mn.sub.0.3 Al.sub.0.3 Single 600 95
89 82 71 59 2.5 259 612 3 ment 1 Roll Method Embodi- LmNi.sub.3.6
Co.sub.0.4 Mn.sub.0.5 Al.sub.0.3 Single 600 97 89 78 75 66 2.3 263
557 2 ment 2 Roll Method Embodi- LmNi.sub.4.0 Co.sub.0.4 Mn.sub.0.3
Cr.sub.0.3 Single 600 96 88 82 79 68 2.3 256 575 3 ment 3 Roll
Method Embodi- LmNi.sub.4.0 3.6Co.sub.0.4 Mn.sub.0.5 Cr.sub.0.5
Single 600 95 91 88 85 71 2.3 264 544 2 ment 4 Roll Method Embodi-
LmNi.sub.4.2 Mn.sub.0.5 Cu.sub.0.8 Single 600 97 91 89 83 62 2.6
261 548 2 ment 5 Roll Method Embodi- LmNi.sub.4.2 Mn.sub.0.8 Single
600 96 92 87 84 73 2.7 271 539 2 ment 6 Roll Method Embodi-
LmNi.sub.4.4 Mn.sub.0.3 Al.sub.0.3 Single 600 95 86 79 76 57 2.5
258 563 2 ment 7 Roll Method Embodi- LmNi.sub.4.0 Co.sub.0.4
Mn.sub.0.3 Fe.sub.0.3 Single 600 97 96 95 87 68 2.4 250 579 2 ment
8 Roll Method Embodi- LmNi.sub.4.0 Co.sub.0.4 Mn.sub.0.3 Si.sub.0.3
Single 600 95 89 86 81 64 2.2 247 548 2 ment 9 Roll Method
__________________________________________________________________________
As apparent from Table 1, since each of the hydrogen-absorbing
alloys for battery according to the examples 1-9 prepared by
rapidly quenching the molten alloys each having the predetermined
composition added with Mn has the columnar structure sufficiently
grown in the alloy structure thereof and a very small amount of
segregations of the elements constituting the alloy, when the
alloys are used as a negative electrode material, an electrode
capacity can be greatly improved without shortening a life.
Further, since a maximum electrode capacity can be achieved by the
small number of charge/discharge cycles or the small number of
activations such as 2-3 cycles (times), the battery characteristics
can be rapidly risen up at an initial time and the manufacturing
cost of the battery can be reduced.
Note, the metal structure of the respective hydrogen-absorbing
alloy powders made in the embodiments 1-9 was analyzed according to
the procedures shown in the following items (A), (B) and the above
item (4) before electrodes were made from the alloy powders.
(A) Burying of Resin
An alloy specimen was taken in an amount of 100 mg and dispersed to
the center of a resin burying frame (made of polypropylene) with a
diameter of 20 mm for SEM specimen.
Next, an epoxy resin (EPO-MIX made by Buller Ltd.) commercially
available as a resin for burying a SEM specimen and a curing agent
were sufficiently mixed and the thus obtained mixed material was
poured into the burying frame and cured. At that time, it was
preferable to preheat the resin to about 60.degree. C. to lower the
viscosity thereof or to remove foams therefrom by evacuating the
resin in a vacuum desiccator after it had been poured into the
frame to improve the intimate contact property of the resin with
the specimen.
(B) Polishing
Next, the specimen buried by the above procedure was polished by a
rotary polishing machine until it was mirror-polished. Since the
specimen of the hydrogen-absorbing alloy was liable to react with
water, it was polished with water-resistant abrasive papers mounted
on the polishing machine rotating at 200 rpm while dropping methyl
alcohol. At that time, the abrasive papers were sequentially
changed to finer ones of #180, #400 and #800. Then, the specimen
was mirror-polished by the diamond paste on the rotary polishing
machine having a felt set thereon, the felt being provided with the
diamond paste whose grain size was made finer in the sequence of 15
microns, 3 microns and 0.25 micron.
The area ratios and minor diameters of the columnar structures of
the hydrogen-absorbing alloys contained in the respective specimens
obtained by the above procedures were measured by the method shown
in the above item (4). As a result, it was confirmed that the
hydrogen-absorbing alloys according to the embodiments 1-9
exhibited values substantially equal to the respective measured
values (Table 1) of the columnar crystals of the alloy contained in
the negative electrode taken from a nickel-hydrogen battery charged
and discharged 10 cycles.
Embodiment 10
An ingot of AB.sub.5 type hydrogen-absorbing alloy in the weight of
200 g was prepared by adjusting the composition thereof by taking
an amount of a law material lost when it was melted into
consideration, so that a rapidly-quenched molten alloy obtained by
being quenched by a molten-metal-rapidly-quenching method had the
composition of LmNi.sub.4.0 Co.sub.0.4. Mn.sub.0.3 Al.sub.0.3 (Lm
is composed of a La-rich misch metal containing Ce: 3 wt %, La: 50
wt %, Nd: 40 wt %, Pr: 5 wt %, and other rare earth elements: 2 wt
%) and the thus obtained ingot was melted in a high frequency
induction heating furnace to prepare a molten alloy. Next, a
flake-shaped rapidly-quenched molten alloy having a thickness of 50
microns was prepared by dropping the thus obtained molten alloy
onto the surface of the cooling roll of the apparatus employing the
single roll method shown in FIG. 1.
The rapidly-quenched molten alloy was subjected to a heat treatment
in an argon atmosphere at the temperature of 300.degree. C. for 4
hours and then pulverized, classified to 200 mesh or less and made
to a hydrogen-absorbing alloy powder for battery [alloy powder
(A)].
On the other hand, a rapidly-quenched molten alloy was prepared in
the same way as the alloy powder (A) and subjected to a heat
treatment for 4 hours at the temperatures set to 150.degree. C.,
200.degree. C., 250.degree. C., 350.degree. C., 400.degree. C.,
500.degree. C., 550.degree. C. and 600.degree. C., respectively,
classified to 200 mesh or less and made to alloy powders (B)-(I).
Further, the alloy powder (A) subjected to a heat treatment for 0.5
hour, 1 hour and 6 hours, respectively was made to powders
(J)-(L).
On the other hand, for comparison, a rapidly-quenched molten alloy
was prepared in the same way as the alloy powder (A), classified to
200 mesh or less and made to an alloy powder (M) without being to a
heat treatment.
Table 2 shows the composition of the respective alloy powders
(A)-(L) having being subjected to the heat treatment and the
composition of the alloy powder (M) not subjected to the heat
treatment.
TABLE 2
__________________________________________________________________________
Heat Treatment Conditions Temperature Time Specimen (.degree.C.)
(Hr) Atmosphere Alloy Composition After Heat Treatment
__________________________________________________________________________
Alloy Powder (A) 300 4 Ar Lm.sub.0.96 Ni.sub.4.00 Co.sub.0.41
Mn.sub.0.29 Al.sub.0.31 Alloy Powder (B) 150 4 Ar Lm.sub.0.97
Ni.sub.4.00 Co.sub.0.41 Mn.sub.0.30 Al.sub.0.30 Alloy Powder (C)
200 4 Ar Lm.sub.0.96 Ni.sub.4.00 Co.sub.0.41 Mn.sub.0.30
Al.sub.0.31 Alloy Powder (D) 250 4 Ar Lm.sub.0.96 Ni.sub.4.00
Co.sub.0.41 Mn.sub.0.30 Al.sub.0.30 Alloy Powder (E) 350 4 Ar
Lm.sub.0.97 Ni.sub.4.00 Co.sub.0.40 Mn.sub.0.30 Al.sub.0.30 Alloy
Powder (F) 400 4 Ar Lm.sub.0.97 Ni.sub.4.00 CO.sub.0.40 Mn.sub.0.29
Al.sub.0.30 Alloy Powder (G) 500 4 Ar Lm.sub.0.96 Ni.sub.4.00
CO.sub.0.40 Mn.sub.0.29 Al.sub.0.31 Alloy Powder (H) 550 4 Ar
Lm.sub.0.93 Ni.sub.4.00 CO.sub.0.41 Mn.sub.0.27 Al.sub.0.30 Alloy
Powder (I) 600 4 Ar Lm.sub.0.91 Ni.sub.4.00 CO.sub.0.42 Mn.sub.0.26
Al.sub.0.31 Alloy Powder (J) 300 0.5 Ar Lm.sub.0.98 Ni.sub.4.00
Co.sub.0.40 Mn.sub.0.30 Al.sub.0.30 Alloy Powder (K) 300 1 Ar
Lm.sub.0.98 Ni.sub.4.00 CO.sub.0.41 Mn.sub.0.30 Al.sub.0.31 Alloy
Powder (L) 300 6 Ar Lm.sub.0.97 Ni.sub.4.00 CO.sub.0.40 Mn.sub.0.30
Al.sub.0.29 Alloy Powder (M) -- -- Ar Lm.sub.0.98 Ni.sub.4.00
Co.sub.0.40 Mn.sub.0.31 AL.sub.0.30
__________________________________________________________________________
As apparent from the result shown in Table 2, the alloy powder (M)
not subjected to the heat treatment exhibits substantially a
desired composition. Further, a large variation in the composition
is not admitted in the alloy powders (A)-(G) and (J)-(L) having
been subjected to the heat treatment in the temperature range of
from 150.degree. C. to 500.degree. C. On the other hand, the alloy
powders (H) and (I) exhibit a large variation of the composition
because they are subjected to the heat treatment at a little higher
temperature and thus the rare earth elements and Mn which are
liable to be evaporated are reduced in a little large amount. Since
the composition of the alloys is liable to be changed when they are
subjected to the heat treatment at a high temperature, the heat
treatment is preferably carried out in a low temperature range.
Next, any one of the thus prepared powders (A)-(M), PTFE powder and
carbon powder were weighed so that they were 95.5 wt %, 4.0 wt %
and 0.5 wt % and kneaded to prepare respective electrode sheets.
The electrode sheets were cut off to a desired size and attached to
a nickel collector under pressure to make hydrogen-absorbing alloy
electrodes. The electrode made of the alloy powders (A)-(M) were
referred to as electrodes (A)-(M), respectively.
Then, the respective hydrogen absorbing-alloy electrodes (A)-(M)
were charged up to 300 mAh/g at a current value of 220 mA per 1 g
of alloy (220 mA/g) and discharged with the above current value
until a potential difference of -0.5 V was achieved with respect to
an Hg/HgO reference electrode and at that time a maximum electrode
capacity was measured. The result of the measurement is shown Table
3.
On the other hand, a paste was prepared by adding a small amount of
CMC (carboxymethyl cellulose) and water to 90 wt % of nickel
hydroxide and 10 wt % of cobalt monoxide and stirring and mixing
them. This paste was filled to a porous nickel member having a
three-dimensional structure and dried and rolled by a roller press
to make a nickel electrode.
Then, AA type nickel-metal hydride batteries were assembled by
combining the above respective hydrogen absorbing-alloy electrodes
(A)-(M) and the nickel electrode. Here, the battery using the alloy
electrode (A) as the negative electrode was referred to as a
battery (A) and in the same way the batteries using the alloy
electrodes (B)-(M) were referred to as batteries (B)-(M),
respectively. Here, the capacity of the respective batteries was
set to 650 mAh as the theoretical capacity of the nickel electrode
and a solution mixed with 7N potassium hydroxide and 1N lithium
hydroxide was used as a battery electrolyte.
Next, the respective batteries (A)-(M) were charged with 650 mA for
1.5 hours and repeatedly subjected to charge/discharge cycles so
that a current was discharged at the current of 1A until the
batteries had a voltage of 1 V and the number of the cycles at
which the capacity of the batteries became 80% of an initial
capacity was measured as a battery life, and the result shown
in
TABLE 3 ______________________________________ Electrode Capacity
of Battery Life Hydrogen-absorbing Alloy (Number of Specimen (mA/g)
Cycles) ______________________________________ Alloy Powder(A) 278
794 Alloy Powder(B) 251 624 Alloy Powder(C) 261 711 Alloy Powder(D)
268 750 Alloy Powder(E) 271 781 Alloy Powder(F) 268 689 Alloy
Powder(G) 260 678 Alloy Powder(H) 231 426 Alloy Powder(I) 220 372
Alloy Powder(J) 251 608 Alloy Powder(K) 269 703 Alloy Powder(L) 274
776 Alloy Powder(M) 259 612
______________________________________
As apparent from the result shown in Table 3, when the alloy
powders subjected to the heat treatment are used, a high electrode
capacity can be obtained as shown in the alloy electrodes (A),
(C)-(G) and (K)-(L). Further, it is confirmed that the batteries
(A), (C)-(G) and (K)-(L) using the alloy powders subjected to the
heat treatment can improve life characteristics to 650-794
cycles.
These phenomena are assumed to be achieved by the following
mechanism. That is, the heat treatment effectively acts in the
alloy electrodes (A), (C)-(G) and (K)-(L) to remove fine crystal
distortions in the alloy structure. As a result, a hydrogen
absorbing capability per unit weight of the alloy is improved.
Therefore, the electrode capacity of these alloy electrodes is
greatly increased as compared with that of the alloy electrode (M)
not subjected to the heat treatment. Further, stress is reduced
when hydrogen is absorbed/released and the life characteristics of
the batteries using these alloy electrodes are greatly improved. In
particular, it is verified that when the alloy powders (A), (D),
(E), (K) and (L) subjected to the heat treatment within the
temperature range of 250.degree.-350.degree. C. for 1 hour or
longer are used, the effect of improvement is particularly
enhanced.
On the other hand, since the alloy powder (B) is subjected to the
heat treatment at the low temperature of 150.degree. C. and the
alloy powder (J) is subjected to the heat treatment for the short
time of 0.5 hour, they can not sufficiently remove the crystal
distortions and can only obtain substantially the same electrode
capacity and battery life as those of the alloy powder (M) not
subjected to the heat treatment.
Further, since the alloy powders (H) and (I) are subjected to the
heat treatment at the excessively high temperature of
550.degree.-600.degree. C., the composition of the alloy powders is
varied by the reduction in the amount of the high volatile Mn and
rare earth elements and the strength of the alloys is lowered by
the secondary recrystallization and thus both the electrode
capacity and battery life of them are lowered.
Embodiment 11
Various rapidly-quenched molten alloys were prepared by the
molten-metal-rapidly-quenching method similar to the embodiment 10
while changing a substituted amount x of Mn. The rapidly-quenched
molten alloys had the composition formula of LmNi.sub.4.3-x
Co.sub.0.4 Mn.sub.x Al.sub.0.3 and the substituted amount of Mn was
changed to 0, 0.1, 0.3, 0.5, 0.8, and 1.2. Further, Lm was La-rich
misch metal similar to that of the embodiment 10. The thus obtained
rapidly-quenched molten alloys were used as they are without being
subjected to a heat treatment to form alloy electrodes. Further,
the alloy electrodes were combined with nickel electrodes to
prepare batteries and the electrode capacity and life of the
batteries were measured.
Further, the respective alloys were subjected to a heat treatment
under the conditions (heated in an Ar atmosphere at the temperature
of 300.degree. C. for 4 hours) which were most effective in the
embodiment 10. Then, various hydrogen-absorbing alloy electrodes
and AA type nickel-metal hydride batteries were prepared in the
same way as the embodiment 10 and the electrode capacity and
battery life thereof were measured. Note, the methods of measuring
electrode capacity and battery life were the same as those of the
embodiment 10.
On the other hand, as a comparative example, an electrode and
battery having the same specification were made by using the ingot
of the hydrogen-absorbing alloy having the composition of Lm
Ni.sub.4.0 Co.sub.0.4 Mn.sub.0.3 Al.sub.0.3 and prepared by a
melting/casing method in place of the
molten-metal-rapidly-quenching method and the electrode capacity
and battery life (cycle life) thereof were measured in the same way
to obtain the result shown in Table 4.
TABLE 4 ______________________________________ Before Heat
treatment After Heat Treatment Substituted Electrode Electrode
Amount of Mn Capacity Cycle Life Capacity Cycle Life (X) (mAh/g)
(No. of Cycles) (mAh/g) (No. of Cycles)
______________________________________ 0.0 221 506 228 597 0.1 240
541 256 693 0.3 259 612 278 794 0.5 263 557 281 762 0.8 268 526 283
717 1.2 275 410 286 475 Ingot(0.3) 250 412 252 406
______________________________________
As apparent from the result shown in Table 4, the effect of the
heat treatment is small in the alloy composition not containing Mn.
On the other hand, the alloy compositions containing Mn obtain a
great improvement effect in both the electrode capacity and battery
life. When, however, the substituted amount of Mn exceeds 1 and
reaches 1.2, the battery life is rapidly dropped and there is no
significant difference between the characteristics of the
conventional alloy ingot made by the melting/casing method and
those of the rapidly-quenched molten alloys.
On the other hand, since the alloy ingot made by the melting/casing
method has the segregations caused to the components constituting
the alloy ingot and located in the wide range of the crystal
structure thereof, the homogeneity of the alloy is not arisen under
the heat treatment conditions limited in the method of this
embodiment (heated at 300.degree. C. for 4 hours). Therefore, the
effect of the heat treatment is not almost realized and thus the
improvement of the battery characteristics is difficult.
Embodiment 12
Six kinds of rapidly-quenched molten alloys were prepared by the
molten-metal-rapidly-quenching method similar to the embodiment 11
while changing a substituted amount x of Mn. The rapidly-quenched
molten alloys had the composition of Lm Ni.sub.4.3-x Co.sub.0.4
Mn.sub.x Cr.sub.0.3 and the substituted amount of Mn was changed to
0, 0.1, 0.3, 0.5, 0.8, and 1.2.
On the other hand, as a comparative example, the ingot of a
hydrogen-absorbing alloy made by the melting/casing method and
having the composition of Lm Ni.sub.4.0 Co.sub.0.4 Mn.sub.0.3
Cr.sub.0.3 was prepared.
Then, alloy electrodes and AA type nickel-metal hydride batteries
were made by using the above various kinds of the rapidly-quenched
molten alloys and the ingot of the hydrogen-absorbing alloy in the
same way as the embodiment 11 and the electrode capacity and
battery life thereof were measured.
Further, the rapidly-quenched molten alloys and the ingot of the
hydrogen-absorbing alloy were subjected to a heat treatment under
the conditions (heated in an Ar atmosphere at the temperature of
300.degree. C. for 4 hours) which were most effective in the
embodiment 10. Then, hydrogen-absorbing alloy electrodes and AA
type nickel-metal hydride batteries were prepared by using the
various heat treated alloys and the electrode capacity and battery
life (cycle life) thereof were measured in the same way as the
embodiment 10, and the result shown in Table 5 was obtained.
TABLE 5 ______________________________________ Before Heat
Treatment After Heat Treatment Substituted Electrode Electrode
Amount of Mn Capacity Cycle Life Capacity Cycle Life (X) (mAh/g)
(No. of Cycles) (mAh/g) (No. of Cycles)
______________________________________ 0.0 218 415 221 436 0.1 243
530 262 603 0.3 256 575 270 756 0.5 264 544 277 741 0.8 270 524 285
705 1.2 277 371 290 449 Ingot(0.3) 258 386 259 403
______________________________________
As apparent from the result shown in Table 5, the alloy composition
not containing Mn does not almost exhibit the improvement effect of
the battery characteristics resulting from the heat treatment. On
the other hand, the electrode capacity is increased and the battery
life is extended by the heat treatment in the alloy components in
which the substituted amount x of Mn is 0.1.ltoreq.x.ltoreq.1.
When, however, the substituted amount x of Mn exceeds 1, it is
confirmed that the battery life is lowered to the level same as
that of the case in which the ingot of cast alloy is used, in the
same way as the embodiment 11.
Embodiment 13
Five kinds of rapidly-quenched molten alloys were prepared by the
molten-metal-rapidly-quenching method similar to the embodiment 11
while changing a substituted amount x of Cu. The rapidly-quenched
molten alloys had the composition of Lm Ni.sub.4.5-x Mn.sub.0.5
Cu.sub.x and the substituted amount x of Cu was changed to 0, 0.3,
0.5, 0.8, and 1.2, respectively.
On the other hand, as a comparative example, a hydrogen-absorbing
alloy ingot made by the melting/casting method and having the
composition of Lm Ni.sub.4.2 Mn.sub.0.5 Cu.sub.0.3 was
prepared.
Then, alloy electrodes and AA type nickel-metal hydride batteries
were made by using the above various kinds of the rapidly-quenched
molten alloys and the ingot of the hydrogen-absorbing alloy in the
same way as the embodiment 11 and the electrode capacity and
battery life thereof were measured.
Further, the rapidly-quenched molten alloys and the ingot of the
hydrogen-absorbing alloy were subjected to the heat treatment in
the Ar atmosphere at the temperature of 300.degree. C. for 4 hours.
Then, hydrogen-absorbing alloy electrodes and AA type nickel-metal
hydride batteries were made by using the various heat treated
alloys and the electrode capacity and battery life (cycle life)
thereof were measured in the same way as the embodiment 10, and the
result shown in Table 6 was obtained.
TABLE 6 ______________________________________ Before Heat
treatment After Heat Treatment Substituted Electrode Electrode
Amount of Cu Capacity Cycle Life Capacity Cycle Life (X) (mAh/g)
(No. of Cycles) (mAh/g) (No. of Cycles)
______________________________________ 0.0 270 526 283 641 0.3 261
548 271 673 0.5 249 573 260 697 0.8 236 563 247 671 1.2 207 372 214
469 Ingot(0.3) 253 309 256 316
______________________________________
In the result shown in Table 6, since the substituted amount of Mn
exhibiting large meritorious effects depending upon the presence or
absence of the heat treatment is given, the variation of the
characteristics of the alloy electrodes and batteries according to
the embodiment 13 is relatively small as compared with the
embodiments 10-12. From the tendency as a whole, however, it can be
confirmed the tendency that both the electrode capacity and battery
life are increased by the execution of the heat treatment.
Further, it is also confirmed that the electrode capacity is
lowered by the increased substituted amount of Cu and that in
particular, when the substituted amount is 1 or more, there is a
tendency that the capacity is rapidly lowered. In addition, it is
also confirmed that when the substituted amount exceeds 1, the
cycle life is rapidly shortened.
According to the hydrogen-absorbing alloys for battery of the
embodiments 10-13, since the molten alloys each having the
predetermined composition containing Mn as an essential component
element are prepared by being rapidly quenched, the alloy
electrodes and batteries having a less amount of segregations, a
high electrode capacity and long life can be formed.
In particular, it is possible to remove internal distortion while
keeping the homogeneity of the alloys by further subjecting the
above rapidly-quenched molten alloys to a heat treatment for 1 hour
or longer at the temperature range of from 200.degree. to
500.degree. C. and preferably at the low temperature range of from
200.degree. to 350.degree. C. Therefore, nickel-metal hydride
batteries having more excellent battery characteristics can be
provided.
Embodiments 1A-9A
Hydrogen-absorbing alloy electrodes according to embodiments 1A-9A
were prepared, respectively under the same conditions as those of
the embodiments 1-9 except that hydrogen-absorbing alloys were made
in such a manner that the respective rapidly-quenched molten alloys
prepared by the single roll method in the embodiments 1-9 were
further subjected to a heat treatment in an Ar gas atmosphere at
the temperature of 300.degree. C. for 4 hours. Further, the
respective hydrogen-absorbing alloy electrodes (negative
electrodes) were combined with a nickel electrode (positive
electrode) to make AA type nickel-metal hydride batteries.
Next, the area ratios of the columnar structures, minor diameters
of the columnar structures, maximum electrode capacity, life (the
number of charge-discharge cycles), and the rising-up of the
initial battery characteristics of the hydrogen-absorbing alloys
were measured under the same conditions as those of the embodiment
1 and the like, and the results shown in Table 7 was obtained.
TABLE 7
__________________________________________________________________________
Characteristics Area Ratio of Columnar Structures Minor Number RPM
of Aspect Ratio Dia of of Manu- Cooling 1:2 1:3 1:4 1:5 Columnar
Electrode Cycle Acti- Specimen facturing Roll or or or or As a
Structures Capacity Life vation No. Alloy Composition Method
(r.p.m) higher higher higher higher whole (.mu.m) (mAh/g) (cycles)
(times)
__________________________________________________________________________
Embodi- LmNi.sub.4.0 Co.sub.0.4 Mn.sub.0.3 Al.sub.0.3 Single 600 95
89 82 75 63 2.5 278 794 3 ment 1A Roll Method Embodi- LmNi.sub.3.6
Co.sub.0.4 Mn.sub.0.5 Al.sub.0.3 Single 600 93 87 80 72 62 2.3 281
762 2 ment 2A Roll Method Embodi- LmNi.sub.4.0 Co.sub.0.4
Mn.sub.0.3 Cr.sub.0.3 Single 600 97 85 79 71 60 2.3 270 756 3 ment
3A Roll Method Embodi- LmNi.sub.3.6 Co.sub.0.4 Mn.sub.0.5
Cr.sub.0.5 Single 600 96 92 86 81 78 2.3 277 741 2 ment 4A Roll
Method Embodi- LmNi.sub.4.2 Mn.sub.0.5 Cu.sub.0.8 Single 600 98 93
87 78 62 2.6 271 673 2 ment 5A Roll Method Embodi- LmNi.sub.4.2
Mn.sub.0.8 Single 600 94 90 82 79 65 2.7 283 548 1 ment 6A Roll
Method Embodi- LmNi.sub.4.4 Mn.sub.0.3 Al.sub.0.3 Single 600 97 89
77 73 45 2.5 274 567 2 ment 7A Roll Method Embodi- LmNi.sub.4.0
Co.sub.0.4 Mn.sub.0.3 Fe.sub.0.3 Single 600 96 91 89 79 68 2.4 262
589 2 ment 8A Roll Method Embodi- LmNi.sub.4.0 Co.sub.0.4
Mn.sub.0.3 Si.sub.0.3 Single 600 93 90 86 77 67 2.2 256 562 2 ment
9A Roll Method
__________________________________________________________________________
As apparent from the result shown in Table 7, since the
hydrogen-absorbing alloys of the respective embodiments having been
subjected to the heat treatment at the relatively low temperature
of 300.degree. C. effectively correct crystal distortions without
damaging the homogeneity thereof, hydrogen can be easily absorbed
and disabsorbed. Therefore, it is verified that the electrode
capacity and the cycle life of the batteries are greatly improved
as compared with the batteries using the respective
hydrogen-absorbing alloys shown in Table 1. In particular, the
battery capacity is increased by about 10%.
Comparative Examples 1A-1D
Molten alloys were rapidly quenched by using a double roll
apparatus having two iron cooling rolls with a diameter of 100 mm
disposed in confrontation to each other as shown in FIG. 2 and
hydrogen-absorbing alloys according to comparative examples 1A-1D
having the final composition of Mm Ni.sub.3.55 Mn.sub.0.4
Al.sub.0.3 Co.sub.0.75 were prepared. Note, the rotation speed of
the cooling rolls was set to 1500 rpm (comparative example 1A),
2000 rpm (comparative example 1B), 2500 rpm (comparative example
1C) and 3000 rpm (comparative example 1D), respectively.
Comparative Examples 2A-2D, 3A and 3B
Molten alloys were rapidly quenched by using a single roll
apparatus having a copper cooling roll with a diameter of 300 mm
and hydrogen-absorbing alloys according to comparative examples
2A-2D and 3A-3B having the final compositions shown in Table 8 were
prepared. Note, the rotation speed of the cooling roll was set to
1000 rpm (comparative example 2A), 1500 rpm (comparative example
2B), 2000 rpm (comparative examples 2B' and 2C), 2500 rpm
(comparative example 2D), 200 rpm (comparative example 3A), and 200
rpm (comparative example 3B), respectively. Note, the rapid
quenching was carried out in an Ar gas atmosphere at 1 atm., the
distance between the extreme end of an injection nozzle for
injecting the molten alloys and the cooling roll was set to 50 mm,
and an injection pressure was set to 0.1 Kgf/cm.sup.2.
Comparative Examples 4-6
Molten alloys were rapidly quenched by using a single roll
apparatus having a copper cooling roll with a diameter of 300 mm
and hydrogen-absorbing alloys according to comparative examples 4-6
having the final compositions shown in Table 8 were prepared,
respectively. Note, the rapid quenching was carried out in an Ar
gas atmosphere at 1 atm., the distance between the extreme end of
an injection nozzle for injecting the molten alloys and the cooling
roll was set to 50 mm, an injection pressure was set to 0.02
Kgf/cm.sup.2, and the rotation speed of the cooling roll was set to
1000 rpm.
Note, as crucibles for preparing the molten alloys, there were used
a crucible made of calcia (comparative example 4), a crucible made
of alumina (comparative example 5) and a crucible made of quartz
(comparative example 6), respectively.
Comparative Examples 7A-7D
A Molten alloy was rapidly quenched by using a single roll
apparatus having a ceramic spray coated roll having a diameter of
300 mm and hydrogen-absorbing alloys according to comparative
examples 7A-7D having the final compositions of Lm Ni.sub.4.2
Co.sub.0.2 Mn.sub.0.3 Al.sub.0.3 were prepared. Note, the rapid
quenching was carried out in an Ar gas atmosphere at 1 atm., the
distance between the extreme end of an injection nozzle for
injecting the molten alloy and the ceramic spray coating roll was
set to 50 mm, an injection pressure was set to 0.1 Kgf/cm.sup.2.
Further, the rotation speed of the ceramic spray coating roll was
set to 1000 rpm (comparative example 7A), 1500 rpm (comparative
example 7B), 2000 rpm (comparative examples 7C), and 2500 rpm
(comparative example 7D), respectively.
Comparative Examples 8A-9B
Molten alloys were rapidly quenched by using a single roll
apparatus having a copper cooling roll with a diameter of 200 mm
and hydrogen-absorbing alloys according to comparative examples
8A-9B having the final compositions shown in Table 9 were prepared.
Note, the rapid quenching was carried out in an Ar gas atmosphere
at 1 atm., the distance between the extreme end of an injection
nozzle for injecting the molten alloys and the cooling roll was set
to 50 mm, an injection pressure was set to 100 mm-H.sub.2 O, and
the rotation speed of the cooling roll was set to 2000 rpm
(comparative examples 8A), 2500 rpm (comparative example 8B), 2000
rpm (comparative examples 9A), and 2500 rpm (comparative example
9B), respectively.
Comparative Examples 10-13
The materials of hydrogen-absorbing alloy powder were adjusted so
that the compositions of alloy ingots had the composition of Mm
Ni.sub.3.2 Co.sub.1.4 Al.sub.0.6 (comparative example 10), Mm
Ni.sub.3.2 Co.sub.1.1 Al.sub.0.7 (comparative example 11), Mm
Ni.sub.3.5 Co.sub.0.7 Al.sub.0.8 (comparative example 12), and Mn
Ni.sub.3.7 Co.sub.0.4 Al.sub.0.9 (comparative example 13),
respectively and were put into a crucible made of alumina, and
molten alloys were prepared by melting them at 1400.degree. C. by a
high frequency induction heating. Next, the molten alloys were cast
in a water-cooled steel casting mold and solidified to make the
ingots of the hydrogen-absorbing alloys according to the
comparative examples 10-13, respectively.
Comparative Examples 14A-14B
The material of a hydrogen-absorbing alloy powder adjusted so that
the composition of an alloy ingot had the composition of
Ni.sub.3.55 Co.sub.0.75 Mn.sub.0.4 Al.sub.0.3 was put into a
crucible made of mullite and melted by being heated to 1500.degree.
C. by a high frequency induction heating coil disposed around the
outside periphery of the crucible to prepare a molten alloy. Next,
the thus obtained molten alloy was cast in a water-cooled steel
casting mold and alloy ingots were prepared with the distance
between casting mold surfaces set to 55 mm (comparative example
14A) and 35 mm (comparative example 14B) at a casting speed of 3
Kg/sec./m.sup.2. Further, the thus obtained alloy ingots were
subjected to a heat treatment in an argon gas atmosphere at
1050.degree. C. for 6 hours to prepare hydrogen-absorbing alloys
according to the comparative examples 14A-14B, respectively.
Comparative Examples 15-20
As comparative examples 15, 16, 18-20, the materials of
hydrogen-absorbing alloy powders adjusted so that the composition
of alloy ingots had the values shown in Table 9 were put into a
crucible made of alumina and heated to 1400.degree. C. by high
frequency induction heating to prepare molten alloys. Note, in the
comparative example 17, the molten alloy was prepared by an arc
melting method. Next, the thus obtained respective molten alloys
were cast in a water-cooled casting mold and solidified to prepare
the ingots of hydrogen-absorbing alloys according to the
comparative examples 15-20. In addition, the ingot of the
hydrogen-absorbing alloy of the comparative example 16 was further
subjected to a heat treatment in an Ar gas atmosphere at
1000.degree. C. for 6 hours.
The thus obtained rapidly-quenched molten alloys or
hydrogen-absorbing alloys according to the comparative examples
1A-20 were pulverized by a stamp mill and classified to 200 mesh or
less to prepare hydrogen-absorbing alloy powders for battery. Next,
hydrogen-absorbing alloy electrodes (negative electrodes) were
prepared by using the respective hydrogen-absorbing alloy powders
for battery in the same procedure as that of the embodiment 1 and
combined with a nickel electrode (positive electrode) to assemble
AA type nickel-metal hydride batteries. Then, the electrode
capacity, the number of charge/discharge cycles (life) and the
number of activations of the electrodes were measured by the same
method as that of the embodiment 1 and the results shown in Tables
8 and 9 were obtained.
TABLE 8
__________________________________________________________________________
Area Ratio of Columnar Structures (%) RPM of Aspect Ratio Cooling
1:2 1:3 1:4 1:5 Manufacturing Roll or or or or As a Specimen No.
Alloy Composition Method (r.p.m) higher higher higher higher whole
__________________________________________________________________________
Comparative Example 1A MmNi.sub.3.55 Mn.sub.0.3 Al.sub.0.3
Co.sub.0.75 Single Roll Method 1500 48 44 35 25 22 Comparative
Example 1B MmNi.sub.3.55 Mn.sub.0.4 Al.sub.0.3 Co.sub.0.75 Single
Roll Method 2000 38 30 25 15 19 Comparative Example 1C
MmNi.sub.3.55 Mn.sub.0.4 Al.sub.0.3 Co.sub.0.75 Single Roll Method
2500 33 28 20 12 12 Comparative Example 1D MmNi.sub.3.55 Mn.sub.0.4
Al.sub.0.3 Co.sub.0.75 Single Roll Method 3000 25 19 13 7 10
Comparative Example 2A MmNi.sub.3.3 CoAl.sub.0.3 Mn.sub.0.5 Single
Roll Method 1000 49 47 38 31 23 Comparative Example 2B MmNi.sub.3.2
CoAl.sub.0.2 Mn.sub.0.5 Single Roll Method 1500 40 33 22 17 19
Comparative Example 2B* Mm.sub.1.4 Ni.sub.3.2 CoAl.sub.0.2
Mn.sub.0.5 Single Roll Method 2000 25 18 15 9 8 Comparative Example
2C MmNi.sub.3.2 CoAl.sub.0.2 Mn.sub.0.5 Single Roll Method 2000 21
18 10 7 8 Comparative Example 2D MmNi.sub.3.2 CoAl.sub.0.2
Mn.sub.0.5 Single Roll Method 2500 9 8 5 2 3 Comparative Example 3A
MmNi.sub.3.2 CoAl.sub.0.2 Mn.sub.0.5 Single Roll Method 200 11 10 8
5 3 Comparative Example 3B Mm.sub.1.4 Ni.sub.3.2 CoAl.sub.0.2
Mn.sub.0.5 Single Roll Method 200 10 9 7 4 2 Comparative Example 4
MmNi.sub.5 Ca.sub.0.01 Single Roll Method 1000 13 12 9 5 6
Comparative Example 5 MmNi.sub.5 Al.sub.0.01 Single Roll Method
1000 10 9 5 2 4 Comparative Example 6 MmNi.sub.5 Si.sub.0.01 Single
Roll Method 1000 15 9 7 3 7 Comparative Example 7A LmNi.sub.4.2
Co.sub.0.2 Mn.sub.0.3 Al.sub.0.3 Single Roll Method 1000 47 41 35
19 21 Comparative Example 7B LmNi.sub.4.2 Co.sub.0.2 Mn.sub.0.3
Al.sub.0.3 Single Roll Method 1500 31 28 21 19 12 Comparative
Example 7C LmNi.sub.4.2 Co.sub.0.2 Mn.sub.0.3 Al.sub.0.3 Single
Roll Method 2000 15 14 8 5 8 Comparative Example 7D LmNi.sub.4.2
Co.sub.0.2 Mn.sub.0.3 Al.sub.0.3 Single Roll Method 2500 7 6 4 2 2
__________________________________________________________________________
Minor Dia of Characteristics Columnar Electrode Cycle Number of
Structures Capacity Life Activation Specimen No. (.mu.m) (mAh/g)
(cycles) (times)
__________________________________________________________________________
Comparative Example 1A 2.2 212 521 7 Comparative Example 1B 2.1 208
501 6 Comparative Example 1C 2.2 210 485 6 Comparative Example 1D
1.8 212 468 6 Comparative Example 2A 2.2 228 515 7 Comparative
Example 2B 2.0 227 497 8 Comparative Example 2B* 1.8 235 415 7
Comparative Example 2C 1.9 229 473 6 Comparative Example 2D 1.6 226
456 7 Comparative Example 3A 2.3 223 459 8 Comparative Example 3B
2.0 235 430 7 Comparative Example 4 1.6 204 317 7 Comparative
Example 5 1.8 208 342 Comparative Example 6 1.7 199 357 7
Comparative Example 7A 2.4 227 480 5 Comparative Example 7B 1.8 223
467 7 Comparative Example 7C 1.7 225 465 6 Comparative Example 7D
1.7 226 448 6
__________________________________________________________________________
TABLE 9
__________________________________________________________________________
Area Ratio of Columnar Structures (%) Minor RPM of Aspect Ratio Dia
of Cooling 1:2 1:3 1:4 1:5 Columnar Manufacturing Roll or or or or
As Structures Specimen No. Alloy Composition Method (r.p.m) higher
higher higher higher whole (.mu.m)
__________________________________________________________________________
Comparative Example 8A LaNi.sub.4.6 Al.sub.0.4 Single Roll Method
2000 12 11 7 5 3 2.4 Comparative Example 8B LaNi.sub.4.6 Al.sub.0.4
Single Roll Method 2500 6 5 3 1> 2 2.2 Comparative Example 9A
LaNi.sub.4.0 Co.sub.0.5 Al.sub.0.4 Single Roll Method 2000 21 18 13
7 9 2.1 Comparative Example 9B LaNi.sub.4.0 Co.sub.0.5 Al.sub.0.4
Single Roll Method 2500 9 4 2 1> 3 2.2 Comparative Example 10
MmNi.sub.3.0 Co.sub.1.4 Al.sub.0.6 Casting Method -- 30 28 25 22 14
2.6 Comparative Example 11 MmNi.sub.3.2 Co.sub.1.1 Al.sub.0.7
Casting Method -- 39 37 31 23 23 2.5 Comparative Example 12
MmNi.sub.3.7 Co.sub.0.4 Al.sub.0.9 Casting Method -- 35 31 29 25 16
2.2 Comparative Example 13 MmNi.sub.3.7 Co.sub.0.4 Al.sub.0.9
Casting Method -- 28 23 17 11 13 2.5 Comparative Example 14A
MmNi.sub.3.55 Co.sub.0.75 Mn.sub.0.4 Al.sub.0.3 Casting Method --
85 43 22 10 5 120 Comparative Example 14B MmNi.sub.3.55 Co.sub.0.75
Mn.sub.0.4 Al.sub.0.3 Casting Method -- 93 47 35 27 11 100
Comparative Example 15 MmNi.sub.3.5 Co.sub.0.7 Al.sub.0.5 Casting
Method -- Max. Crystal Grain Size 150 .mu.m Comparative Example 16
MmNi.sub.3.7 Al.sub.0.5 Fe.sub.0.7 Cu.sub.0.1 Casting Method --
Max. Crystal Grain Size 200 .mu.m Comparative Example 17
MmNi.sub.3.8 Co.sub.0.2 Al.sub.0.5 Casting Method -- Max. Crystal
Grain Size 100 .mu.m Comparative Example 18 LaNi.sub.4.5 Al.sub.0.5
Casting Method -- Max. Crystal Grain Size 170 .mu.m Comparative
Example 19 LaNi.sub.4.7 Al.sub.0.3 Casting Method -- Max. Crystal
Grain Size 180 .mu.m Comparative Example 20 LaNi.sub.4.2 Co.sub.0.3
Sn.sub.0.2 Al.sub.0.3 Casting Method -- Max. Crystal Grain Size 170
__________________________________________________________________________
.mu.m Characteristics Electrode Cycle Number of Capacity Life
Activation (mAh/g) (cycles) (times)
__________________________________________________________________________
Comparative Example 225 376 6 Comparative Example 223 360 7
Comparative Example 200 462 6 Comparative Example 205 447 7
Comparative Example 200 497 8 Comparative Example 207 525 8
Comparative Example 215 509 7 Comparative Example 219 475 8
Comparative Example 260 432 6 Comparative Example 262 441 7
Comparative Example 200 520 9 Comparative Example 210 70 8
Comparative Example 208 440 7 Comparative Example 220 385 7
Comparative Example 233 370 5 Comparative Example 207 390 6
__________________________________________________________________________
As apparent from the result shown in Table 8 and Table 9, in each
of the hydrogen-absorbing alloys shown in the comparative examples
1A-20, the area ratio of the columnar structures in a metal
structure is reduced as compared with that of the embodiments shown
in Tables 1 and 7. Therefore, it is confirmed that the electrode
capacity using these alloys is low and the battery life (cycle
life) represented by the number of charge/discharge cycles is also
short.
Further, in the electrodes of the comparative examples, the number
of charge/discharge cycles (the number of activations) necessary to
obtain a maximum electrode capacity is 5-9 times which are twice or
more the 2-3 times of the embodiments, and thus it is confirmed
that the initial rising-up property of the electrodes is also
low.
In particular, the columnar structures are not sufficiently grown
even in the hydrogen-absorbing alloys according to the comparative
examples 1A-1D, 2A-2D, 3A-3B, 7A-7D each added with Mn in the same
way as the embodiments and the ratio of equi-axed crystals are
increased in some cases, and thus the battery characteristics are
lowered as compared with the embodiments shown in Tables 1 and
7.
FIG. 27 is a photograph taken by an electron microscope (SEM) to
show the metal structure of the cross section of the
hydrogen-absorbing alloy according to the comparative example 7B.
In FIG. 27, the lower portion of the cross section is a quenched
side in contact with the cooling roll and the upper portion thereof
is a free side. Columnar structures are grown from the quenched
side toward the free side, whereas an equi-axed crystal structure
is formed in the portion near to the free side because of an
insufficient quenching rate. The area ratio of the columnar
structures in an entire crystal structure is 50% or less.
Further, in the batteries using the hydrogen-absorbing alloys
according to the comparative examples 4, 5, 6 8A-9B to which Mn is
not contained, an electrode capacity is about 200-225 mAh/g and
further a battery life is as low as 300-400 cycles.
Further, it is found that the hydrogen-absorbing alloys according
to the comparative examples 10-13 made by being gradually cooled by
the casting method have a small area ratio of columnar structures,
low electrode capacity and a bad initial rising-up property of
electrode.
On the other hand, it is found that the hydrogen-absorbing alloys
containing Mn according to the comparative examples 14A-14B made by
being gradually cooled by the casting method have sufficiently
grown columnar structures, but a battery life is short and an
initial rising-up property of electrode is bad because the minor
diameter of the columnar structures is as large as 100-120 microns,
although a relatively high electrode capacity can be obtained.
Further, the hydrogen-absorbing alloys without containing Mn
according to the comparative examples 15-20 made by being gradually
cooled by the casting method have a crystal particle size increased
to about 100-200 microns, and thus an alloy strength is lowered and
a battery life is short.
As described above, when the respective embodiments shown in Tables
1 and 7 are compared with the respective comparative examples shown
in Tables 8 and 9, it is found that the hydrogen-absorbing alloys
and batteries according to the embodiments satisfy all of the three
leading characteristics of the electrode capacity, battery life and
initial rising-up property.
Next, the relationship between the crystal particle size and the
battery life of hydrogen-absorbing alloy will be described with
reference to the following embodiments and comparative
examples.
Embodiments 14-17 and Comparative Examples 21-22
The material of an alloy powder adjusted so that the composition of
a hydrogen-absorbing alloy was Lm Ni.sub.4.0 Co.sub.0.4 Mn.sub.0.3
Al.sub.0.3 was put into a an crucible and heated by high frequency
induction heating to prepare a molten alloy.
As embodiments 14-17, respective hydrogen-absorbing alloys were
made by rapidly quenching the molten alloy by using a single roll
apparatus having a cooling roll with a diameter of 300 mm. The
material and the rotating speed of the cooling roll were set as
shown in Table 10.
On the other hand, as comparative examples 21-22, respective
hydrogen-absorbing alloys were made in such a manner that the above
molten alloy was put into a water-cooled copper casting mold and
cast with the distance between casting mold surfaces set to 45 mm
(comparative example 21) and 80 mm (comparative example 22).
Hydrogen-absorbing alloy electrodes (negative electrodes) were
prepared by crushing the hydrogen-absorbing alloys according to the
embodiments 14-17 and comparative examples 21-22 under the same
conditions as those of the embodiment 1 and further combined with
nickel electrodes (positive electrodes) to prepare nickel-metal
hydride batteries. Then, the number of charge/discharge cycles of
the respective batteries was measured under the same conditions as
the embodiment 1, and the result shown in Table 10 was
obtained.
TABLE 10
__________________________________________________________________________
Dia. of Crystal Material of RPM of Grains Cycle Manufacturing
Cooling Roll or Cooling Roll (Minor Dia.) Life Specimen No. Method
Casting Mold (r.p.m.) (.mu.m) (cycles)
__________________________________________________________________________
Embodiment 14 Single Roll Method Cu 600 2.5 612 Embodiment 15
Single Roll Method Fe 600 9 593 Embodiment 16 Single Roll Method Fe
+ 5 .mu.m thick 600 19 543 Cr plated Embodiment 17 Single Roll
Method Si.sub.3 N.sub.4 300 28 511 Comparative Casting Method Cu --
83 448 Example 21 Comparative Casting Method Cu -- 105 440 Example
22
__________________________________________________________________________
Further, FIG. 28 shows the relationship between the crystal
particle size and the cycle life. Note, in the embodiments 14-17
having grown columnar structures, the crystal particle size is
shown by a minor diameter.
As apparent from the result shown in Table 10 and FIG. 28, it is
found that as the crystal particle size is increased, a battery
life tends to be rapidly shortened. To achieve the battery life
(the number of charge/discharge cycles) of 500 cycles or more, the
crystal particle size of the hydrogen-absorbing alloy used to
negative electrode must be set to 30 microns or less.
Embodiments 18A-18C and Comparative Example 23
As embodiments 18A-18C, the material of an alloy powder adjusted so
that the composition of a hydrogen-absorbing alloy was Lm
Ni.sub.0.4 Co.sub.0.4 Mn.sub.0.3 Al.sub.0.3 was put into a crucible
made of Ti-boride and heated by high frequency induction heating to
prepare a molten alloy. On the other hand, the molten alloy was
rapidly quenched at an average quenching rate of
2400.degree.-3100.degree. C./sec. by using a single roll apparatus
having a cooling roll (cooling water temperature: 20.degree. C.)
with a diameter of 300 mm made of copper to make the
hydrogen-absorbing alloys of the embodiments 18A-18C.
The quenching rate was calculated as follows. When the molten alloy
was injected from an nozzle, the molten alloy was rapidly quenched
and solidified because the heat thereof was conducted to the
cooling roll, and then the rapidly quenched and solidified alloy
was exfoliated from the cooling roll and driven away from the
cooling roll: when the cooling roll was rotated at about 600 rpm,
the rapidly quenched alloy was exfoliated and driven away from the
cooling roll after it had been completely solidified, whereas when
the cooling roll was rotated at about 1000 rpm, the rapidly
quenched alloy was exfoliated and driven away from the cooling roll
before it had been completely solidified because the cooling roll
had a strong centrifugal force. Although the moving distance of the
molten alloy from the time at which it came into contact with the
cooling roll to the time at which it was exfoliated therefrom
changed depending upon the composition of the alloy, when the
injecting state of the molten metal of the embodiment 18A
(rotation: 600 rpm) was photographed by a high-speed video camera,
that distance corresponded to 1/8 rotation of the cooling roll.
When the time necessary to move this distance was assumed to be a
quenching time, the quenching time was 1/80 sec. Then, since the
time necessary to drop the temperature of the molten alloy from an
injecting temperature (1380.degree. C.) to a solidifying point
(1350.degree. C.) was 1/80 sec., the quenching rate was about
2400.degree. C./sec. Since, however, there was a dispersion in the
position where the alloy was exfoliated, an average quenching rate
was employed.
On the other hand, as a comparative example 23, the material of an
alloy powder adjusted so that the composition of a
hydrogen-absorbing alloy was Lm Ni.sub.3.5 Co.sub.0.7 Mn.sub.0.4
Zn.sub.0.1 Al.sub.0.3 was put into a crucible made of alumina and
heated by high frequency induction heating to prepare a molten
alloy. On the other hand, the molten alloy was rapidly quenched at
a quenching rate of 1150.degree. C./sec. by using a single roll
apparatus having an iron cooling roll (cooling water temperature:
50.degree. C., rpm: 200) with a diameter of 300 mm to make the
hydrogen-absorbing alloy.
Then, hydrogen-absorbing alloy electrodes (negative electrodes)
were prepared by crushing the hydrogen-absorbing alloys according
to the embodiments 18A-18C and comparative example 23 under the
same conditions as those of the embodiment 1 and further combined
with nickel electrodes (positive electrodes) to prepare
nickel-metal hydride batteries. Then, the number of
charge/discharge cycles of the respective batteries was measured
under the same conditions as the embodiment 1 as well as the
hydrogen-absorbing alloys were taken out and the ratio of the
columnar structures in the metal structure of each of the
hydrogen-absorbing alloys was measured, and the result shown in
Table 11 was obtained.
TABLE 11
__________________________________________________________________________
Area Ratio of Characteristics Columnar Structures (%) Minor No. of
Average Aspect Ratio Dia of Charge/ Quenching 1:2 1:3 1:4 1:5
Columnar Electrode Discharge No. of Rate or or or or As a
Structures Capacity Cycles Activation Specimen No. (.degree.C./sec)
higher higher higher higher Whole (.mu.m) (mAh/g) (cycles) (times)
__________________________________________________________________________
Embodiment 18A 2400 96 90 83 75 63 1.9 261 610 3 Embodiment 18B
2700 94 91 87 77 65 1.7 263 612 2 Embodiment 18C 3100 95 93 84 78
63 1.7 262 612 2 Comparative 1150 20 18 9 7 2.2 5 255 480 4 Example
23
__________________________________________________________________________
As apparent from the result shown in Table 11, the alloys according
to the embodiments 18A-18C prepared at the increased peripheral
speed of the cooling roll and the high quenching rate have
sufficiently grown columnar structures and exhibit excellent
battery characteristics as compared with the comparative example 23
quenched at the low quenching rate.
Embodiment 19
Next, the composition ratio x of a rare earth element A and the
other elements B in the hydrogen-absorbing alloy composed AB.sub.x
was changed in the range of from 4.4 to 6.1 and the effect of the
change of the composition ratio x on the electrode capacity and the
number of charge/discharge cycles was investigated. That is,
various hydrogen-absorbing alloys represented by Lm (Ni.sub.0.8
Co.sub.0.08 Mn.sub.0.06 Al.sub.0.06).sub.x were made by rapidly
quenching a molten alloy at a quenching rate of 2400.degree.
C./sec.
When the composition ratio x=5, this hydrogen-absorbing alloy
corresponded to the hydrogen-absorbing alloy of the first
embodiment. The thus obtained respective hydrogen-absorbing alloy
were processed in the same way as the embodiment 1 to make negative
electrodes, and the electrode capacity of the negative electrodes
was measured as well as the negative electrodes were combined with
a positive electrode to assemble nickel-metal hydride batteries and
the number of charge/discharge cycles thereof were measured, and
the result shown in FIG. 31 was obtained.
As apparent from the result shown in FIG. 31, it is confirmed that
the electrode capacity is 240 mAh/g or more within the range of
4.5.ltoreq.x.ltoreq.6.0 limited in the present invention and the
excellent battery characteristics such as the number of
charge/discharge cycles of 500 times or more are achieved. Further,
it is also verified that the excellent battery characteristics of
the electrode capacity of 250 mAh/g or more and the number of
charge/discharge cycles of 550 times or more are achieved in the
more preferable range (4.6.ltoreq.x.ltoreq.5.8) of the composition
ratio x.
As described above, the hydrogen-absorbing alloy for battery
according to this embodiment is an AB.sub.5 type alloy containing
Mn as an essential element, which can provide a negative electrode
material having a high electrode capacity and excellent cycle life
and initial characteristics used to secondary nickel-metal hydride
battery because the components constituting the hydrogen-absorbing
alloy have a less amount of segregations. The above alloy can be
obtained by rapidly quenching a molten alloy at a quenching rate of
1800.degree. C./sec. or higher.
Further, internal distortions can be removed while maintaining the
homogeneity of the alloy by subjecting the above rapidly-quenched
molten alloy to a heat treatment at a relatively low temperature of
about 200.degree.-500.degree. C., by which a nickel-metal hydride
battery excellent in battery characteristics can be provided.
Further, other embodiments of the present invention will be
described together with comparative examples.
Embodiment 20
(Manufacture of Alloy)
When a hydrogen-absorbing alloy having a stoichiometric composition
of LmNi.sub.4.0 Co.sub.0.4 Mn.sub.0.3 Al.sub.0.3 (Lm: La-rich misch
metal) was made, materials were weighed in the weight ratio for
achieving a non-stoichiometric composition without destroying the
composition ratio of Ni, Co, Mn, Al by reducing the amount of Lm
when the materials were weighed. Next, these materials were melted
in a high frequency induction heating furnace and accommodated to
the ladle 2 of the aforesaid manufacturing apparatus employing the
single roll method shown in FIG. 1 and a molten hydrogen-absorbing
alloy 3 was dropped onto a cooled cooling roll 5 from the ladle 2
so that six kinds of flake-shaped hydrogen-absorbing alloys 6
(specimens a-f) were obtained.
The composition of the thus obtained alloys is shown in Table 12.
Further, these alloys were pulverized in a ball mill and then
classified through a sieve of 200 mesh to make hydrogen-absorbing
alloys for electrode.
TABLE 12 ______________________________________ Alloy Composition
Symbol of Alloy Specimen (AB.sub.x)
______________________________________ a AB.sub.5.0 b .sub.
AB.sub.5.05 c AB.sub.5.5 d AB.sub.5.8 e AB.sub.6.0 f AB.sub.6.2
______________________________________
Further, the thus obtained hydrogen-absorbing alloys (specimens a-f
in Table 12) were measured by an X-ray diffraction in accordance
with an internal standard method. As a result, it is confirmed that
95 vol % or more of the specimens a-e was a single phase composed
of AB.sub.x. Further, it is also found that 10 vol % or more of a
phase (second phase) having a composition other than AB.sub.x is
grown in the specimen f.
(Manufacture of Electrode)
The respective hydrogen-absorbing alloy powders,
polytetrafluoroethylene (PTFE) powder and ketchen black were
weighed to 95.5 wt %, 4 wt % and 0.5 wt %, respectively and then
stirred and mixed by a cutter mill until the PTFE was made to
fibers. The thus obtained cotton-like mixture was scattered onto a
nickel metal net and rolled by a roller press to make
hydrogen-absorbing alloy electrodes (negative electrodes).
(Manufacture of Battery)
A paste was prepared by adding a small amount of CMC (carboxymethyl
cellulose) and 50 wt % of water to 90 wt % of nickel hydroxide and
10 wt % of cobalt monoxide and stirring and mixing them. That paste
was filled to a porous nickel member having a three-dimensional
structure and dried and rolled by a roller press to make nickel
electrodes (positive electrodes).
Groups of electrodes were arranged in such a manner that each of
the above nickel electrodes having a theoretical capacity of 1.1 Ah
was combined with each of the above hydrogen-absorbing alloy
electrodes and they were wound through a separator composed of a
non-woven fabric. The groups of the electrodes were inserted into
AA type battery cans (containers), and each of the cans was filled
with 30 wt % potassium hydroxide aqueous solution and sealed by the
terminal plate of a positive electrode having a safety valve
operating at a pressure of 15 Kg/cm.sup.2 to assemble secondary
nickel-metal hydride batteries shown in FIG. 3. Note, in the
secondary nickel-metal hydride battery, a negative
electrode/positive electrode ratio was set to 1.8 and the amount of
battery electrolyte to the hydrogen-absorbing alloy electrode was
set to 1.1 ml/Ah. The negative electrode/positive electrode ratio
means the capacity ratio of the alloy in an uncharged state in the
battery discharged state of the hydrogen-absorbing alloy electrode.
That is, this means the ratio of the capacity of the
hydrogen-absorbing alloy electrode to that of the nickel electrode,
excluding the discharge reserve in the hydrogen-absorbing alloy
electrode produced by the oxidation and the like of the cobalt
monoxide in the nickel electrode when the battery is charged for
the first time after has been assembled.
The cycle life of the respective secondary batteries was evaluated
under the conditions that the batteries were charged at 1.1 A for
1.5 hours and a discharge cycle for discharging at 1 A was repeated
until a battery voltage was 0.8 V. A cycle life was determined by
the number of cycles when the battery capacity was reduced to 50%
of an initial capacity.
Table 13 shows the result of the evaluation.
TABLE 13 ______________________________________ Charge/Discharge
Battery No. Alloy Composition Cycle (cycles)
______________________________________ 1 AB.sub.5.0 250 2 .sub.
AB.sub.5.05 360 3 AB.sub.5.5 520 4 AB.sub.5.8 490 5 AB.sub.6.0 450
6 AB.sub.6.2 120 ______________________________________
From Table 13, it is found that when non-stoichiometric composition
is achieved, the cycle life is extended and when x of AB.sub.x
exceeds 6, the cycle life is rapidly shortened. It is found from
the result of the X-ray diffraction analysis effected to the
AB.sub.6.2 alloy that the second phase is grown in an amount of 10
vol % more, and from the result of the observation by EPMA that a
large amount of La and Mn is segregated to the particle boundaries
thereof, from which it is clear that the creation of the second
phase and the segregations to the particle boundaries greatly
shorten the alloy life.
Comparative Example 24
A hydrogen-absorbing alloy having the same composition as that of
the embodiment 20 was made by a conventional casting method.
Materials weighed so that the same composition as that of the
embodiment 20 was achieved were put into a crucible and melted in a
high frequency induction heating furnace and then poured into a
casting mode made of iron to make the alloy. The thus obtained
alloy was made to hydrogen-absorbing alloy electrodes in the same
way as the embodiment 20 and subjected to a powder X-ray
diffraction analysis to observe the creation of a second phase.
Table 14 shows the result of the analysis, wherein .smallcircle.
shows the creation of the single phase, x shows the creation of the
second phase and .DELTA. shows the possibility of the creation the
second phase in an amount of 10 wt % or more, although not
clear.
TABLE 14 ______________________________________ Alloy Composition
Result of Analysis ______________________________________
AB.sub.5.0 .smallcircle. .sub. AB.sub.5.05 .DELTA. AB.sub.5.5 x
AB.sub.5.8 x AB.sub.6.0 x AB.sub.6.2 x
______________________________________
As apparent from Table 14, it is found that since a quenching rate
is slow in the casting method, when the alloy is composed of the
non-stoichiometric composition, the second phase is liable to be
created. Although this may be improved by the use of a water-cooled
casting mold or reducing a casting thickness, it is very difficult
to stably form the single phase up to the vicinity of the
AB.sub.5.5 where a performance is greatly improved.
From the above mentioned facts, it is supposed that the
hydrogen-absorbing alloy made by the molten-metal-rapidly-quenching
method is suitable as described in the embodiment 20.
Although the single roll method is described in the embodiment 20
as a method of easily and stably making the hydrogen-absorbing
alloy having the stoichiometric composition, the rotating disk
method described in FIG. 32, double roll method described in FIG.
2, gas atomizing method described in FIG. 33 and other rotating
nozzle method and the like as other molten-metal-rapidly-quenching
methods also can stably provide the hydrogen-absorbing alloy having
the stoichiometric composition in the same way.
Although the embodiment 20 uses Ni, Co, Mn and Al as the elements
constituting B of the hydrogen-absorbing alloy represented by
AB.sub.x, the same result can be obtained by using, for example,
Si, Fe, Cr, Cu and the like.
Embodiment 21
A hydrogen-absorbing alloy in an amount of 500 g having the
composition of Lm Ni.sub.4.0 Co.sub.0.4 Mn.sub.0.3 Al.sub.0.3 (Lm:
La-rich misch metal) was melted in a high frequency induction
furnace and accommodated to the ladle 2 of the aforesaid
manufacturing apparatus employing the single roll method shown in
FIG. 1 and a molten hydrogen-absorbing alloy 4 was dropped onto a
roll (a cooling roll) 5 from the ladle 2 so that a flake-shaped
hydrogen-absorbing alloy 6 was made. The alloy 6 was pulverized in
a ball mill and then classified through a 200 mesh sieve to make
hydrogen-absorbing alloy powders for making electrode. Next, the
hydrogen-absorbing alloy powders, PTFE powder and ketchen black
were weighed to 95.5 wt %, 4 wt % and 0.5 wt %, respectively and
then stirred and mixed by a cutter mill until the PTFE was made to
fibers. The thus obtained cotton-like mixture was scattered onto a
nickel metal net and rolled by a roller press to make
hydrogen-absorbing alloy electrodes (negative electrodes).
Further, a paste separately prepared by adding a small amount of
CMC and 50 wt % of water to 90 wt % of nickel hydroxide and 10 wt %
of cobalt monoxide and stirring and mixing them. That paste was
filled to a nickel porous member having a three-dimensional
structure and dried and rolled by a roller press to make nickel
electrodes (positive electrodes).
The hydrogen-absorbing alloy electrodes and nickel electrodes made
by the aforesaid method were used to assemble 30 kinds of AA type
secondary nickel-metal hydride batteries which had the above
negative electrode/positive electrode ratio and the amount of a
battery electrolyte to the hydrogen-absorbing alloy electrodes
shown in Table 15. The capacity of these secondary batteries was
set to 650 mAh as the theoretical capacity of the nickel electrode
and the battery electrolyte was composed of a solution mixed with
7N potassium hydroxide and 1N lithium hydroxide. Further, a safety
valve operating at 18 Kg/cm.sup.2 was used. Note, the portion in
Table 15 in which battery symbols are not shown indicates that
since an electrode volume is so large that a predetermined amount
of the battery electrolyte cannot be poured into the battery
cans.
TABLE 15 ______________________________________ Negative Electrode/
Positive Electrode Ratio 0.5 1.1 1.5 2.0 2.5 3.0
______________________________________ Amount of Battery 0.2 A1 A2
A3 A4 A5 A6 Electrolyte to 0.4 B1 B2 B3 B4 B5 B6 Hydrogen-absorbing
0.8 C1 C2 C3 C4 C5 C6 Alloy Electrode 1.5 D1 D2 D3 D4 D5 -- (ml/Ah)
1.8 E1 E2 E3 E4 -- -- 2.0 F1 F2 F3 -- -- --
______________________________________
The number of cycles of each of the respective secondary batteries,
which belonged to the group of the symbol A (A1, A2, A3, A4, A5,
A6), symbol group B (B1, B2, B3, B4, B5, B6), group of the symbol C
(C1, C2, C3, C4, C5, C6), group of the symbol D (D1, D2, D3, D4,
D5), group of the symbol E (E1, E2, E3, E4) and group of the symbol
F (F1, F2, F3), was investigated until the capacity of the
batteries was reduced to 50% of an initial capacity under the
conditions that the batteries were charged at 650 mA for 1.5 hours
and then a discharge cycle discharging at 1 A was repeated until
the voltage of the batteries was 0.8 V. FIG. 34 shows the result of
the investigation. In FIG. 34, .quadrature. shows the
characteristics of the group A batteries in Table 15, + shows the
characteristics of the group B batteries in Table 15, .diamond.
shows the characteristics of the group C batteries in Table 15,
.DELTA. shows the characteristics of the group D batteries in Table
15, x shows the characteristics of the group E batteries in Table
15, and .gradient. shows the characteristics of the group F
batteries in Table 15, respectively.
As apparent from FIG. 34, it is found that the secondary batteries
having the negative electrode/positive electrode ratio of 0.5 have
very short cycle life in all the amounts of the battery
electrolyte. This is because that the arrangement of the batteries
is limited by the hydrogen-absorbing alloy electrode and thus a
capacity is only 0.5 time the positive electrode and the internal
pressure of the batteries is abruptly increased at an initial
charge-discharge cycle to the pressure at which the safety valve is
operated and the shortage of the battery electrolyte is caused
because the electrolyte is discharged through the safety valve.
This fact is confirmed by the measurement of the internal pressure
of the batteries and the check of disassembled batteries executed
separately.
On the other hand, the secondary batteries having the negative
electrode/positive electrode ratio of 1.1 or higher achieve the
cycle life of 150 times or more under the charge/discharge
conditions unless the amount of the battery electrolyte is
excessively large or excessively small. This cycle life is a value
satisfying the cycle life of 500 cycles which can be practically
employed in a secondary battery in usual charge/discharge
conditions and is supposed to be a sufficient cycle life. Even if
the negative electrode/positive electrode ratio is 1.1 or higher,
however, when the amount of the battery electrolyte is 0.2 ml/Ah to
2.0 ml/Ah, the cycle life is about 50 cycles which are not
satisfactory. This is because that the amount of the battery
electrolyte is insufficient at an initial charge/discharge cycle
under the condition of 0.2 ml/Ah and the internal pressure of the
batteries is increased to the pressure valve operating pressure at
the end of the charging under the condition of 2.0 ml/Ah and thus
the battery electrolyte was gradually shorted. This fact is
confirmed by the measurement of the internal pressure of the
batteries executed separately.
Further, the secondary batteries belonging to the groups A-F in
Table 15 were charged under the severe conditions of 650 mA for 5
hours and the internal pressure thereof was measured to investigate
the internal pressure characteristics of the batteries which had a
large effect on the battery life. FIG. 35 shows the result of the
investigation. Note, in FIG. 35, .quadrature. shows the
characteristics of the batteries of the group A in Table 15, +
shows the characteristics of the batteries of the group B in Table
15, .diamond. shows the characteristics of the batteries of the
group C in Table 15, .DELTA. shows the characteristics of the
batteries of the group D in Table 15, x shows the characteristics
of the batteries of the group E in Table 15, and .gradient. shows
the characteristics of the batteries of the group F in Table 15,
respectively.
It is found from FIG. 35 that the battery internal pressure of all
of the secondary batteries having the negative electrode/positive
electrode ratio of 0.5 and secondary batteries having the amount of
the battery electrolyte of 2 ml/Ah reaches 18 Kg/cm.sup.2 at which
the safety valve is operated.
When the weights of these secondary batteries were measured before
and after the test, a loss of weight supposed to be caused by the
leakage of the battery electrolyte was observed in a maximum amount
of 300 mg and in a minimum amount of 40 mg.
The reason why the internal pressure of all the secondary batteries
having the amount of the battery electrolyte of 0.2 ml/Ah is low in
this experiment is that since the batteries had the very small
amount of the battery electrolyte and the internal resistance in
the batteries was abruptly increased, the output range of a
constant current power supply used for charging was exceeded to
prevent a sufficient charging. From this fact, it is found that the
secondary batteries having the amount of the battery electrolyte of
0.2 ml/Ah cannot be practically used.
It is found from the above experiment for measuring the battery
life and battery internal pressure that the negative
electrode/positive electrode ratio must be 1.1 or higher and the
amount of the battery electrolyte must be within the range of
0.4-1.8 ml per 1 Ah of the hydrogen-absorbing alloy electrode.
Further, there is admitted a tendency that the battery internal
pressure is increased from the vicinity of the point where the
negative electrode/positive electrode ratio exceeds 2. This is
supposed to be caused by that since all the secondary batteries
have the uniform capacity of 650 mAh, the volume occupied by the
group of the electrodes is increased in the batteries having the
large negative electrode/positive electrode ratio and thus the
space in the batteries is reduced, from which it found that the
amount of the battery electrolyte and negative electrode/positive
electrode ratio have interdependent values and they cannot be
independently determined in the design of battery.
Next, groups of electrodes each having a maximum capacity capable
of being accommodated in an AA type battery can were made when a
certain negative electrode/positive electrode ratio was given,
batteries were assembled with battery electrolytes having various
conditions and poured into them, and the capacity of each of the
batteries was measured in the initial sound state of the batteries
in order to confirm a high capacity as a large feature of a
secondary nickel-metal hydride battery. FIG. 36 shows the result of
the measurement. Note, in FIG. 36,.quadrature. shows the
characteristics of the secondary battery in which the amount of the
battery electrolyte to the hydrogen-absorbing alloy electrode is
0.2 ml/Ah, + shows the characteristics of the secondary battery in
which the amount of the battery electrolyte is 0.4 ml/Ah, .diamond.
shows the characteristics of the secondary battery in which the
amount of the battery electrolyte is 0.8 ml/Ah, .DELTA. shows the
characteristics of the secondary battery in which the amount of the
battery electrolyte is 1.5 ml/Ah, x shows the characteristics of
the secondary battery in which the amount of the battery
electrolyte is 1.8 ml/Ah, and .gradient. shows the characteristics
of the secondary battery in which the amount of the battery
electrolyte is 0.2 ml/Ah, respectively.
It is found from FIG. 36 that since the battery capacity of the
battery having the negative electrode/positive electrode ratio of
0.5 is determined by the hydrogen-absorbing alloy electrode, the
capacity is low.
Further, the amount of the battery electrolyte of 0.2 ml/Ah is not
sufficient to cause a battery reaction, the battery capacity is
also low. In the batteries except the aforesaid ones, as the
negative electrode/positive electrode ratio is increased, the
capacity of the nickel electrode capable of being assembled in the
volume of the same group of electrodes is reduced, and thus the
battery capacity is also reduced.
Then, it is found that when the negative electrode/positive
electrode ratio exceeds 2.0, only a capacity similar to or less
than that of the currently used high capacity type nickel-cadmium
battery is obtained.
Although a higher capacity can be achieved by increasing the
activating material density in electrode even if the negative
electrode/positive electrode ratio is 2.0 or higher, in this case
not only the manufacture of the electrode is difficult but also an
oxygen reducing rate is greatly lowered at the end of charging as
well as large current discharging characteristics are also greatly
lowered and thus the battery using the electrode cannot be
practically used.
It is found from this experiment that the negative
electrode/positive electrode ratio must be 2.0 or less to enable
secondary nickel-metal hydride battery to exhibit the feature of
high capacity without sacrificing the battery characteristics.
Although the embodiment 21 describes in detail the example of the
AA type battery arranged by the hydrogen-absorbing alloy electrode
as the activating material obtained from the hydrogen-absorbing
alloy made by rapidly quenching a molten alloy by the single roll
method, non-sintered nickel electrode and battery electrolyte
composed of the mixture of 7N potassium hydroxide and 1N lithium
hydroxide, the present invention is not limited to this method.
For example, the hydrogen-absorbing alloy obtained by the rotating
disc method described in FIG. 32, double roll method described in
FIG. 2, gas atomizing method described in FIG. 33 and the like as
the molten-metal-rapidly-quenching method exhibited the same
characteristics as those obtained by the single roll method of this
embodiment 21.
Further, with respect to the method of making electrode, the
feature of the present invention can be also exhibited by a
so-called paste type electrode made by filling or coating a kneaded
material of hydrogen-absorbing alloy powder, kneading agent and
water with or to a collector and then drying and rolling the same.
The above hydrogen-absorbing alloy powder may be a powder obtained
by being pulverized by the absorption/release of hydrogen in
addition to the powder made by mechanically pulverizing a
hydrogen-absorbing alloy made by the molten-metal-rapidly-quenching
method. In particular, when the single roll method or double roll
method is employed, a flake-shaped hydrogen-absorbing alloy can be
obtained under wide manufacturing conditions. Since this
flake-shaped alloy is generally thin and can be pulverized
relatively easily, it is possible that the alloy is pulverized only
from several hundreds of microns to a few millimeters prior to the
manufacture of electrode and further pulverized in the rolling
process for making the electrode together with the electrode.
The specific surface area of the hydrogen-absorbing alloy can be
reduced by this processing in the manufacture of the electrode, the
hydrogen-absorbing alloy can be protected from surface oxidation
and pollution in the manufacturing process of the alloy as well as
even the possibility of firing of the hydrogen-absorbing alloy in
the manufacturing process can be reduced, and thus an operation can
be executed in a safe environment.
The battery electrolyte may be, for example, a 8N potassium
hydroxide solution or a battery electrolyte mixed with a sodium
hydroxide solution when necessary.
Further, with respect to a battery size, the same effect can be
obtained even in the battery size of, for example, 4/5A type or A
type in addition to the AA type.
As described above in detail, according to this embodiment, there
can be provided a hydrogen-absorbing alloy which is less
deteriorated when used as a negative electrode activating material
of a secondary alkaline battery and has a long cycle life. Further,
according to this embodiment, a secondary nickel-metal hydride
battery can be provided which has a high capacity and long life and
can be made at a low cost by using the a hydrogen-absorbing alloy
electrode with a less amount of deterioration and limiting the
amount of battery electrolyte and electrode capacity ratio.
Next, the dispersion of Mn concentration in a hydrogen-absorbing
alloy and the effect of the particle size of segregated Mn
particles on battery characteristics will be described below with
reference to the following embodiments and comparative
examples.
Embodiments 22-23 and Comparative Embodiments 25-27
Lm, Ni, Co, Mn, Al were weighed, previously taking the amount of
them lost when they were melted into consideration, so that these
materials had the composition of Lm Ni.sub.4.3 Co.sub.0.1
Mn.sub.0.5 Al.sub.0.1 (Lm: La-rich misch metal) when made to an
alloy.
Next, these materials were melted in a high frequency induction
furnace and flowed into a usual casting mold to provide the alloy
for a comparative example 25 and flowed into a water-cooled casting
mold to provide the alloy for a comparative example 26 with a
thickness of 10 mm. Further, in the single roll method shown in
FIG. 1, a copper roll with a diameter of 300 mm was used as the
cooling roll 5 and rotated at 600 rpm in vacuum, the distance
between the injection nozzle 4 and the cooling roll 5 was set to 50
mm and an injection pressure was set to 0.1 Kgf/cm.sup.2 so that
the alloy with a thickness of about 100 microns for a comparative
example 27 was provided. Further, the injection pressure was set to
0.05 Kgf/cm.sup.2 so that the alloy with a thickness of about 50
microns for an embodiment 22 was provided. Further, the injection
pressure was set to 0.02 Kgf/cm.sup.2 so that the alloy with a
thickness of about 20 microns for an embodiment 23 was provided.
Thus, three kinds of the specimens of the flake-shaped alloys were
made. In addition to the above, a hydrogen-absorbing alloy powder
with an average particle size of 20 microns was prepared as an
embodiment 24 by the inert gas atomizing method shown in FIG.
33.
The distribution of Mn concentration in these alloy specimens was
investigated by the following method.
1) Burying of Resin
An alloy specimen was taken in an amount of 100 mg and dispersed to
the center of a resin burying frame (made of polypropylene) with a
diameter of 20 mm for SEM specimen.
Next, an epoxy resin (EPO-MIX made by Buller Ltd.) commercially
available as a resin for burying SEM specimen and a curing agent
were sufficiently mixed and then the thus obtained mixed material
was poured into the burying frame and cured. At that time, it was
further preferable to preheat the resin to about 60.degree. C. to
lower the viscosity thereof or to remove foams therefrom by
evacuating the resin in a vacuum desiccator after it had been
poured into the frame to improve the intimate contact property of
the resin with the specimen.
2) Polishing
Next, the specimen buried by the above procedure was polished by a
rotary polishing machine until it was mirror-polished. Since the
specimen of the hydrogen-absorbing alloy was liable to react with
water, it was polished with water-resistant abrasive papers mounted
on the polishing machine rotating at 200 rpm while dropping methyl
alcohol. At that time, the abrasive papers were sequentially
changed to finer ones of #180, #400 and #800. Then, the specimen
was mirror-polished by the diamond paste on the rotary polishing
machine having a felt set thereon, the felt being provided with the
diamond paste whose grain size was made finer in the sequence of 15
microns, 3 microns and 0.25 micron.
3) Observation by EPMA
Next, each of these specimens were set to an EPMA (Model V6, made
by Shimazu Seisakusho). First, a region where Mn is not contained
at all (e.g., a specimen table) was observed to perform a mapping
observation and the intensity value of characteristic X-rays was
recorded when a Mn concentration was 0. Next, the specimen was
moved so that it was located at the center of a visual field and
the entire image of the distribution of the Mn concentration was
grasped by the mapping observation. Note, in the measurement, a
magnification was set such that an observation region of
20.times.20 microns was within the visual field. Further, at that
time, a caution was taken so that the edge of the specimens was not
located in the visual field. Then, the visual field was vertically
and horizontally divided into 100 portions to form 10000 pieces of
unit regions and the EPMA was set to measure the intensity value of
the X-rays for each region.
The distribution of the characteristic X-ray intensity within the
observed surface obtained by the above observation was corrected by
the previously determined intensity value within the region in
which Mn was not contained.
The average value of the characteristic X-ray intensity within the
observed surface was determined by simply averaging the
characteristic X-ray intensity corresponding to the respective unit
regions obtained as described above. Further, the maximum value of
the characteristic X-ray intensity was used as a maximum value.
Further, the ratio of the maximum value to the average value of the
X-ray intensity was calculated.
The above analysis was performed 10 times while changing the visual
field.
Table 16 shows the result of the analysis. Further, Table 16 also
shows the quenching rates when the specimens were made, the
quenching rates (cooling speeds) being measured by the same method
as that in the embodiments 18A-18C.
TABLE 16 ______________________________________ Max. Average Max.
Intensity Quenching Intensity Intensity Value/Average Rate Specimen
No. Value Value Intensity Value (.degree.C./S)
______________________________________ Comparative 68 48 1.42 80
Example 25 Comparative 70 51 1.37 250 Example 26 Comparative 64 47
1.36 1000 Example 27 Embodiment 22 60 49 1.22 2000 Embodiment 23 60
52 1.15 3000 Embodiment 24 60 50 1.20 50000
______________________________________
It is found from Table 16 that the ratio of the maximum intensity
value to the average intensity value is lowered in the sequence of
the comparative example 25, comparative example 26, comparative
example 27, embodiment 22, embodiment 24, and embodiment 23.
Next, Table 17 shows the result of observation of the crystal types
of the respective specimens observed by the method described in the
embodiments 1-9.
TABLE 17 ______________________________________ Area Ratio of
Columnar Structures (%) Minor Aspect Ratio Dia of 1:2 1:3 1:4 1:5
Columnar Specimen or or or or As a Structures No. higher higher
higher higher Whole (.mu.m) ______________________________________
Comparative Max. Crystal Grain Size 180 .mu.m Example 25
Comparative 19 10 3 2 9 9.5 Example 26 Comparative 39 28 21 10 11
1.9 Example 27 Embodiment 71 69 62 51 33 2.7 22 Embodiment 95 89 82
89 69 2.5 23 Embodiment Max. Crystal Grain Size 10 .mu.m 24
______________________________________
In Table 17, the reason why no numerical value is shown in the
comparative example 25 and embodiment 24 is that an equi-axed
crystal structure is observed on the entire cross section of the
comparative example 25 and a columnar structure is not recognized
therein, and further in the embodiment 24 a structure other than
the columnar structure is also formed, although the kind of the
structure cannot be recognized.
When Table 16 is compared with Table 17, the reason why there is
the correspondence between the ratio of columnar structures and the
uniformity of the Mn distribution in the comparative examples 26,
27 and the embodiments 22 and 23 is as described below. That is,
when the quenching rate is high as in the case of the embodiments
22, 23, the molten alloy is rapidly quenched and solidified and the
solidification progresses in one direction from the quenched
surface and columnar structures are liable to be grown so that the
ratio of the columnar structures is increased.
Further, it is supposed that since the molten alloy is rapidly
solidified, particular elements cannot exist in a molten state for
the period of time necessary for the particular elements to form
segregations in particles and particle boundaries and thus even the
element such as Mn liable to produce an irregular distribution and
segregations is solidified while keeping the uniformity of
distribution.
Further, it is supposed that the reason why the embodiment 24 is
excellent in the uniformity of Mn distribution regardless of that
no columnar structure is recognized therein is that since the
quenching rate is also very high and particular elements cannot
exist in a molten state for the time necessary for the elements to
form segregations in particles and particle boundaries, even the
element such as Mn liable to produce an irregular distribution and
segregations is solidified while keeping the uniformity of
distribution.
Next, electrodes were made by using these specimens by using the
following procedure. First, the above alloys were pulverized in a
ball mill and the particles thereof larger than 200 mesh were
removed by a 200 mesh sieve to provide hydrogen-absorbing alloy
powders. Next, the hydrogen-absorbing alloy powders, PTFE powder
and ketchen black were weighed to 95.5 wt %, 4 wt % and 0.5 wt %,
respectively and then stirred and mixed by a cutter mill until the
PTFE was made to fibers. The thus obtained cotton-like mixture was
dispersed onto a nickel metal net and rolled by a roller press to
make hydrogen-absorbing alloy electrodes.
Each of these electrodes was bound with a sintered type nickel
electrode through a nylon separator and immersed into a 8N
potassium hydroxide solution and the cycle life thereof was
evaluated through charging/discharging.
The evaluation was carried out by repeating a charge/discharge
cycle under the conditions that charging was effected for 1 hour at
a current of 220 mA per 1 g of the alloy in the electrode and
discharging was effected also at the current of 220 mA until -0.5 V
was achieved to a Hg/HgO electrode. A cycle life was determined by
the number of cycles when an electrode capacity was lowered to 50%
of an initial electrode capacity. The evaluation was performed at
20.degree. C. FIG. 37 shows the change of capacity as the cycle
proceeds and Table 18 shows the cycle life.
TABLE 18 ______________________________________ Max. Intensity
Value/ Cycle Life Specimen No. Average Intensity Value (cycles)
______________________________________ Comparative Example 25 1.42
320 Comparative Example 26 1.37 350 Comparative Example 27 1.36 400
Embodiment 22 1.22 700 Embodiment 23 1.15 740 Embodiment 24 1.20
620 ______________________________________
As shown in the above, it is confirmed that in the comparative
examples 25-27 in which the X-ray intensity ratio to the Mn
concentration in the hydrogen-absorbing alloy is about 1.2, the
cycle life of the electrode is about 350 cycles, whereas in the
embodiments 22-24 in which an X-ray intensity ratio is 1.3 or less
which is within the range of the present invention, the cycle life
in the electrode is greatly extended to 620-740 cycles.
Next, to investigate the performance in actual batteries, batteries
was made in the following procedure by using these alloys and the
cycle life thereof was evaluated.
The hydrogen-absorbing alloy electrodes were made by the same
method as that of the above electrodes for evaluation.
Note, a caution was taken so that the amount of the
hydrogen-absorbing alloy in the electrodes was set to 9 g.+-.0.2
g.
A nickel electrode was made in such a manner that a paste was
prepared by adding a small amount of CMC and 50 wt % of water to 90
wt % of nickel hydroxide and 10 wt % of cobalt monoxide and
stirring and mixing them and filled with a nickel porous member
having a three-dimensional structure and dried and rolled by a
roller press. At that time, a capacity calculated from the weight
of the nickel hydroxide in the electrode was set to 1.1 Ah.
Each of the thus made hydrogen-absorbing alloy electrodes and the
nickel electrode were combined and wound separated by a
polypropylene non-woven fabric to provide groups of electrodes.
Each of the groups of the electrodes was inserted into a battery
can, and the can was filled with a solution mixed with 7N potassium
hydroxide and 1N lithium hydroxide as an battery electrolyte and
sealed by the terminal plate of a positive electrode having a
safety valve operating at a pressure of 15 Kg/cm.sup.2 to assemble
a test battery shown in FIG. 3.
The cycle life was evaluated by using these batteries by repeating
a charge/discharge cycle under the conditions that each battery was
charged at 1.1 A for 1.5 hours and then discharged at 1 A until a
battery voltage was 0.8 V. A cycle life was determined by the
number of cycles when the battery capacity was lowered to 50% of an
initial capacity. A test temperature was 25.degree. C. FIG. 38
shows the change of capacity as the cycle proceeds and Table 19
shows the cycle life.
Note, at this time, it is confirmed that the uniformity of Mn
distribution can be also observed in the electrodes made to battery
by the following procedure in the same way as the case of the
alloy.
1) Discharge of Battery
First, batteries in which a charge/discharge cycles were not
progressed after they had been made were prepared as specimens. As
a charge/discharge cycle progressed, the Mn uniformity in an alloy
was changed and thus it was preferable that the number of the
charge/discharge cycle of the batteries used as the specimens was
10 cycles or less, if possible.
These batteries were discharged in a battery state at a current of
110 mA until a voltage was 0.8 V and then remained they were for 10
hours with a resistance of 10K.OMEGA. connected between the
electrodes of each of the batteries so that they were perfectly
discharged. The reason why the batteries were discharged in the two
steps was to prevent firing caused by remaining hydrogen.
2) Washing of Electrode
After the completion of the discharge, the batteries were
disassembled and the hydrogen-absorbing alloy electrodes were taken
out therefrom. The thus taken-out alloy electrodes were
sufficiently washed with pure water and then perfectly dried in a
vacuum drier for the purpose of removing hydrogen remaining in the
electrodes and preventing the insufficient solidification of the
resin caused by the battery electrolyte and the strength by which
the specimens were attached to the resins from being lowered.
3) Burying of Resin
Each of the dried electrodes was cut off to the size of about 10
mm.times.5 mm and dispersed to the center of a resin burying frame
(made of polypropylene) with a diameter of 20 mm for SEM specimen.
Next, an epoxy resin (EPO-MIX made by Buller Ltd.) commercially
available as a resin for burying a SEM specimen and a curing agent
were sufficiently mixed and then the thus obtained mixed material
was poured into the burying frame and cured. At that timer it was
further preferable to preheat the resin to about 60.degree. C. to
lower the viscosity thereof or to remove foams therefrom by
evacuating the resin in a vacuum desiccator after it had been
poured into the frame to improve the intimate contact property of
the resin with the specimen.
4) Polishing
Next, the specimen buried by the above procedure was polished by a
rotary polishing machine until it was mirror-polished. Since the
specimen of the hydrogen-absorbing alloy was liable to react with
water, it was polished with water-resistant abrasive papers mounted
on the polishing machine rotating at 200 rpm while dropping methyl
alcohol. At that time, the abrasive papers were sequentially
changed to finer ones of #180, #400 and #800. Then, the specimen
was mirror-polished by the diamond paste on the rotary polishing
machine having a felt set thereon, the felt being provided with the
diamond paste whose grain size was made finer in the sequence of 15
microns, 3 microns and 0.25 micron.
5) Observation by EPMA
Next, the average value and maximum value of the X-ray intensity
corresponding to the Mn concentration in the specimens were
measured in the same procedure as above by using an EPMA.
Table 20 shows the result of the measurement, wherein substantially
the same numerical values as those in Table 16 are obtained and
thus it is confirmed that observation is sufficiently possible even
after the alloys have been made to electrode and battery.
TABLE 19 ______________________________________ Cycle Life Specimen
No. (cycles) ______________________________________ Comparative
Example 25 95 Comparative Example 26 130 Comparative Example 27 170
Embodiment 22 480 Embodiment 23 510 Embodiment 24 410
______________________________________
TABLE 20 ______________________________________ Max. Intensity
Average Max. Intensity Value/ Specimen No. Value Intensity Average
Intensity Value ______________________________________ Comparative
69 48 1.44 Example 25 Comparative 68 50 1.36 Example 26 Comparative
67 50 1.34 Example 27 Embodiment 22 57 47 1.21 Embodiment 23 59 50
1.18 Embodiment 24 60 50 1.20
______________________________________
As shown in the above, in the batteries using the alloys of the
comparative examples 25-27 having the low uniformity of the Mn
distribution, the increase of the internal pressure of the
batteries caused by the deterioration of the hydrogen-absorbing
alloys is admitted from the relatively initial stage of the
charge/discharge cycles, in the same way as the result of the above
evaluation of electrodes. Since the battery electrolyte is flown
out from the safety valve by the increase of the internal pressure,
battery capacities are reduced and thus the cycle life of only
about 130 cycles is obtained. Whereas, the internal pressure of the
batteries using the alloys of the embodiments 22-24 having improved
uniformity is relatively gradually increased, and as a result the
cycle life of 410 to 510 cycles is successfully obtained in the
batteries which greatly exceeds 300 cycles as the practical life of
a secondary battery. Thus, it is confirmed that the cycle life can
be greatly increased even in actual batteries by setting the
uniformity of the Mn distribution within the range of the present
invention.
Embodiments 25, 26 and Comparative Examples 28, 29
Lm, Ni, Co, Mn, Al were weighed, previously taking the amount of
them lost when they were melted into consideration, so that these
materials had the composition of Lm Ni.sub.4.0 Co.sub.0.4
Mn.sub.0.3 Al.sub.0.3 (Lm: La-rich misch metal) when made to an
alloy.
Next, these materials were melted in a high frequency induction
furnace and flowed into a usual casting mold to make the alloy for
a comparative example 28. Further, in the single roll method shown
in FIG. 1, a copper roll with a diameter of 300 mm was used as the
cooling roll and rotated at 800 rpm in vacuum, the distance between
the injection nozzle and the cooling roll was set to 50 mm and an
injection pressure was set to 0.1 Kgf/cm.sup.2 so that the alloy
with a thickness of about 100 microns was prepared for a
comparative example 29. Further, the injection pressure was set to
0.02 Kgf/cm.sup.2 so that a flake-shaped alloy specimen with a
thickness of 20 microns was made for an embodiment 25. In addition
to the above, a hydrogen-absorbing alloy powder with an average
particle size of 20 microns was prepared as an embodiment 26 by the
inert gas atomizing method shown in FIG. 33.
The size of the segregated Mn in these alloy specimens was
investigated by the method described below. First, the alloy
specimens were sealed in synthetic resin by the same procedure as
the embodiments 22-24 and mirror-polished by a polisher.
Next, each of the specimens was set to SEM (Model ABT-55 made by
ABT) with EDX (made by KEVEX) and observed. First, a mapping
observation of the Mn and other elements constituting the
hydrogen-absorbing alloy was performed by the EDX and the locations
where Mn was independently segregated were searched. Since,
however, the EDX had a lower resolution as compared with that of
the EPMA and it was difficult to directly determine the size of the
segregations from the result of the mapping observation, the points
having high Mn concentration determined by the EDX had to be
observed by the SEM to correctly find the size of the segregations
of Mn. That is, since the points where the concentration of Mn and
other elements was different from the alloy composition were
observed as the points where the intensity of a reflected electron
beam was different from that of the alloy portion in the SEM, the
reason why the intensity was different was determined by the EDX
and then the size of a region was determined from the result of
observation by the SEM.
Table 21 shows the result of observation of the size of
segregations of Mn of the comparative examples 28,29 and
embodiments 25 and 26 performed by the SEM and EDX. Note, 10 visual
fields were observed for each specimen to prepare Table 21.
TABLE 21 ______________________________________ Max. Diameter of
Segregated Mn Specimen No. (.mu.m)
______________________________________ Comparative Example 28 1.52
Comparative Example 29 0.71 Embodiment 25 0.41 Embodiment 26 0.10
______________________________________
Next, the crystal types of the respective specimens were observed
by the method described in the embodiments 1-9 and Table 22 shows
the result of the observation.
TABLE 22 ______________________________________ Area Ratio of
Columnar Structures (%) Minor Aspect Ratio Dia of 1:2 1:3 1:4 1:5
Columnar Specimen or or or or As a Structures No. higher higher
higher higher Whole (.mu.m) ______________________________________
Comparative Max. Crystal Grain Size 160 .mu.m Example 28
Comparative 17 11 3 1 10 9.0 Example 29 Embodiment 88 72 62 51 89
2.7 25 Embodiment Max. Crystal Grain Size 10 .mu.m 26
______________________________________
In Table 22, the reason why no numerical value is shown in the
comparative example 28 and embodiment 26 is that an equi-axed
crystal structure is observed on the entire surface of the
comparative example 28 and a columnar structure is not recognized
therein, and further in the embodiment 26 a structure other than
the columnar structure is also formed, although the kind of the
structure cannot be clearly recognized.
When Table 21 is compared with Table 21, the reason why there is a
correspondence between the ratio of columnar structures and the
size of Mn segregations in the comparative examples 29 and
embodiments 25 is as described below. That is, when a quenching
rate is high as in the case of the comparative example 29 and
embodiments 25, the molten alloy is rapidly quenched and
solidified, and the solidification is liable to progress in a given
direction from the quenched surface and thus columnar structures
are liable to be grown so that the ratio of the columnar structures
is increased. Further, it is supposed that since the molten alloy
is rapidly solidified, particular elements cannot exist in a molten
state until the time at which the particular elements form
segregations in the particles and particle boundaries, and even if
the segregations are made, they are difficult to be grown, and thus
even the element such as Mn liable to be segregated cannot be made
to a large segregated material. Further, the reason why in the
embodiment 26 the Mn segregations have a small size regardless of
that no columnar structure is admitted is supposed to be that since
a quenching rate is also very high, particular elements cannot
exist in a molten state until the time at which the particular
elements form segregations in the particles and particle
boundaries, and even if the segregations are made, they are
difficult to be grown, and thus even the element such as Mn liable
to be segregated cannot be made to a large segregated material.
Next, these specimens each in an amount of 20 g were sealed in a
sealed vessel (volume: 50 cc) with a gas introduction pipe and the
sealed vessel absorbed hydrogen pressurized to 10 Kg/cm.sup.2
through the pipe while the vessel was immersed into cooled water of
10.degree. C. and then the hydrogen was released from the sealed
vessel by connecting the pipe to a vacuum pump while the vessel was
immersed in hot water of 60.degree. C. After the repetition of the
absorption and release of the hydrogen 1000 times, the distribution
of particles of the specimens were measured by a laser scattering
type particle distribution measuring instrument (made by Seishin
Kigyo Co.) Table 23 shows the result of the measurement.
TABLE 23 ______________________________________ Particle Size of
Hydrogen-absorbing Specimen No. Alloy (.mu.m)
______________________________________ Comparative Example 28 9
Comparative Example 29 13 Embodiment 25 27 Embodiment 26 17
______________________________________
It is found from Table 23 that in the embodiment 25 having the Mn
segregations whose size is small and within the range of the
present invention, even if the absorption/release of hydrogen is
repeated 1000 times, the average size of 35 microns prior to the
test is reduced only to 27 microns and the average size of 20
microns in the embodiment 26 is reduced only to 17 microns, and
thus it is found that the pulverization caused by the
absorption/release of hydrogen to the hydrogen-absorbing alloy is
restricted. On the other hand, it is confirmed that the initial
average size of 35 microns in the comparative examples 28 and 29 is
reduced to 9 microns and 13 microns, respectively.
Next, a cycle life test was executed to electrodes to confirm the
effect of the difference in the pulverizing behavior confirmed in
the above test on the actual electrode characteristics.
Here, electrodes were made by the following procedures. First, the
above alloys were pulverized in a ball mill and the particles
thereof larger than 200 mesh were removed by a 200 mesh sieve to
provide hydrogen-absorbing alloy powders.
Next, the hydrogen-absorbing alloy powders, PTFE powder and ketchen
black were weighed to 95.5 wt %, 4 wt % and 0.5 wt %, respectively
and then stirred and mixed by a cutter mill until the PTFE was made
to fibers. The thus obtained cotton-like mixture was scattered onto
a nickel metal net and rolled by a roller press to make
hydrogen-absorbing alloy electrodes.
Each of the electrodes was bound to a sintered type nickel
electrode through a nylon separator and immersed into a 8N
potassium hydroxide solution and the cycle life thereof was
evaluated through charging and discharging.
The evaluation was carried out by repeating a charge/discharge
cycle under the conditions that charging was effected for 1 hour at
a current of 220 mA per 1 g of the alloy in the electrode and
discharging was effected also at the current of 220 mA per 1 g of
the alloy until -0.5 V was achieved to a Hg/HgO electrode. The
number of cycles was determined by the cycle life when an electrode
capacity was lowered to 50% of an initial electrode capacity. The
evaluation was performed at 20.degree. C. Table 24 shows the cycle
life.
TABLE 24 ______________________________________ Cycle Life Specimen
No. (cycles) ______________________________________ Comparative
Example 28 330 Comparative Example 29 380 Embodiment 25 725
Embodiment 26 650 ______________________________________
When the result of Table 24 is compared with that of Table 23, it
is found that the comparative examples 28, 29 in which
pulverization is liable to be progressed has a shorter life than
the embodiments 25, 26 in which pulverization is restricted and it
is confirmed that the cycle life can be remarkably extended by the
restriction of the progress of pulverization.
Note, at this time, it is confirmed that the size of Mn
segregations in the alloy made to electrode by the following
procedures also can be observed in the same way as alloy as a
single body.
1) Discharge of Electrode
First, electrodes in which a charge/discharge cycles were not
progressed after they had been made were prepared as specimens. As
the charge/discharge cycle progressed, the state of Mn segregations
in the alloys was changed and thus it was preferable that the
number of the charge/discharge cycle of the electrodes used as the
specimens was 10 cycles or less, if possible. These electrodes were
discharged at a current of 0.1 A until -0.5 V was achieved to a
Hg/HgO electrode so that they were perfectly discharged. The reason
why the they were perfectly discharged was to prevent firing caused
by remaining hydrogen.
2) Washing of Electrode
After the completion of the discharge, the hydrogen-absorbing alloy
electrodes were sufficiently washed with pure water and then
perfectly dried in a vacuum drier for the purpose of removing
hydrogen remaining in the electrodes and preventing the
insufficient solidification of resins caused by the battery
electrolyte and the strength by which the specimens were attached
to the resins from being lowered.
3) Burying of Resin
Each of the dried electrodes was cut off to the size of about 10
mm.times.5 mm and dispersed to the center of a SEM resin burying
frame (made of polypropylene) with a diameter of 20 mm for SEM
specimen. Next, an epoxy resin (EPO-MIX made by Buller Ltd.)
commercially available as a resin for burying a SEM specimen and a
curing agent were sufficiently mixed and then the thus obtained
mixed material was poured into the burying frame and cured. At that
time, it was further preferable to preheat the resin to about
60.degree. C. to lower the viscosity thereof or to remove foams
therefrom by evacuating the resin in a vacuum desiccator after it
had been poured into the frame to improve the intimate contact
property of the resin with the specimen.
4) Polishing
Next, the specimen buried by the above procedure was polished by a
rotary polishing machine until it was mirror-polished. Since the
specimen of the hydrogen-absorbing alloy was liable to react with
water, it was polished with water resistant abrasive papers mounted
on the polishing machine rotating at 200 rpm while dropping methyl
alcohol. At this time, the abrasive papers were sequentially
changed to finer ones of #180, #400 and #800. Then, the specimen
was mirror-polished by the diamond paste on the rotary polishing
machine on which a felt was set with the grain size of the diamond
paste made finer in the sequence of 15 microns, 3 microns and 0.25
micron.
5) Observation by EDX
Next, each of the specimens was set to SEM (Model ABT-55 made by
ABT) with EDX (made by KEVEX) and observed. First, a mapping
observation of the Mn and other elements constituting the
hydrogen-absorbing alloy was performed by the EDX and the locations
where Mn was independently segregated were searched. Since,
however, the EDX had a lower resolution as compared with that of
the EPMA and it was difficult to directly determine the size of the
segregations from the result of the mapping observation, the points
with high Mn concentration determined by the EDX had to be observed
by the SEM to correctly find the size of the segregations of Mn.
That is, since the points where the concentration of Mn and other
elements was different from the alloy composition were observed as
the points where the intensity of a reflected electron beam was
different from that of the alloy portion in the SEM, the reason why
the intensity was different was determined by the EDX and then the
size of a region was determined from the result of observation by
the SEM.
The above analysis was performed 10 times while changing the visual
field.
Although Ni, Co, Mn, Al were used as the elements constituting B in
the above embodiment, the same effect could be obtained even if a
part thereof was substituted for Cr, Si, Fe, Cu, Ag, Pd, Sn, In,
Ga, Ge, Ti, Zr, Zn or the like.
As described above in detail, the corrosion resistance of the
alloys against a thick alkaline solution as a battery electrolyte
is improved as well as the pulverization caused by the
expansion/shrinkage due to the absorption/release of hydrogen can
be suppressed by setting the maximum value of Mn concentration in
the hydrogen-absorbing alloys containing Mn which is very effective
to increase the capacity of the hydrogen-absorbing alloys to 1.3
times or less the average value thereof or setting the maximum
diameter of Mn segregated in the alloys to 0.1 micron or less,
whereby the life of the hydrogen-absorbing alloys can be extended.
As a result, the life of the secondary nickel-metal hydride battery
employing these alloys can be extended. Consequently, the
hydrogen-absorbing alloys according to the present invention have a
very large industrial value.
As described above, since the first to third hydrogen-absorbing
alloys for battery of the present invention are composed of an
AB.sub.5 type alloy containing Mn as an essential element, the
hydrogen-absorbing alloys can provide a negative electrode material
having a high capacity and excellent cycle life and initial
characteristics when used to secondary alkaline battery.
Further, when the alloys are subjected to a heat treatment at a
relatively low temperature of about 200.degree.-500.degree. C., the
internal distortion of the alloys can be removed while keeping the
homogeneity thereof, and thus the alloys can provide nickel-metal
hydride battery having more excellent battery characteristics.
Further, the method of manufacturing hydrogen-absorbing alloys
according to the present invention can provide a negative electrode
material having a high capacity and excellent cycle life and
initial characteristics for secondary nickel-metal hydride
battery.
Further, the fourth hydrogen-absorbing alloy according to the
present invention can form electrode having a less amount of
deterioration and long cycle life when used as a negative electrode
activating material.
Further, the secondary nickel-metal hydride battery according to
the present invention has a high capacity and long life and can be
made at low cost.
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